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A cooling fluid pump is for a cooling a marine engine. The cooling fluid pump comprises a pump chamber that contains an impeller. An upstream inlet passage supplies cooling fluid to the pump chamber. A downstream outlet passage discharges cooling fluid from the pump chamber. An impeller shaft rotate

A cooling fluid pump is for a cooling a marine engine. The cooling fluid pump comprises a pump chamber that contains an impeller. An upstream inlet passage supplies cooling fluid to the pump chamber. A downstream outlet passage discharges cooling fluid from the pump chamber. An impeller shaft rotates the impeller, causing flow of cooling fluid through the pump chamber from the inlet passage to the outlet passage. A drain passage connects the pump chamber to the inlet passage such that at least when the impeller is not rotating, the drain passage drains cooling fluid that settles by gravity in the pump chamber back to the inlet passage.

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1. A cooling fluid pump for a cooling a marine engine, the cooling fluid pump comprising a body having a pump chamber that contains an impeller, an upstream inlet passage that supplies cooling fluid to the pump chamber, and a downstream outlet passage that discharges cooling fluid from the pump cham

1. A cooling fluid pump for a cooling a marine engine, the cooling fluid pump comprising a body having a pump chamber that contains an impeller, an upstream inlet passage that supplies cooling fluid to the pump chamber, and a downstream outlet passage that discharges cooling fluid from the pump chamber; an impeller shaft disposed in the body, wherein the impeller shaft rotates the impeller, wherein rotation of the impeller causes flow of cooling fluid through the pump chamber from the upstream inlet passage of the body to the downstream outlet passage of the body; and a drain passage in the body, wherein the drain passage connects the pump chamber to the upstream inlet passage; wherein at least when the impeller is not rotating, the drain passage drains cooling fluid that settles by gravity in the pump chamber back to the upstream inlet passage. 2. A cooling fluid pump for a cooling a marine engine, the cooling fluid pump comprising a pump chamber that contains an impeller, an upstream inlet passage that supplies cooling fluid to the pump chamber; a downstream outlet passage that discharges cooling fluid from the pump chamber; an impeller shaft that rotates the impeller, wherein rotation of the impeller causes flow of cooling fluid through the pump chamber from the upstream inlet passage to the downstream outlet passage; and a drain passage that connects the pump chamber to the upstream inlet passage; wherein at least when the impeller is not rotating, the drain passage drains cooling fluid that settles by gravity in the pump chamber back to the upstream inlet passage, wherein the drain passage has a drain inlet opening and a drain outlet opening, and wherein the drain inlet opening is located in the pump chamber and the drain outlet opening is located in the upstream inlet passage. 3. The cooling fluid pump according to claim 2, wherein the drain inlet opening has a circular cross-section. 4. The cooling fluid pump according to claim 2, wherein operation of the impeller increases pressure at the downstream outlet passage and decreases pressure at the upstream inlet passage; and further comprising a check valve that prevents cooling fluid from draining to the upstream inlet passage through the drain passage during said operation of the impeller. 5. The cooling fluid pump according to claim 4, wherein the check valve is a pressure activated check valve that opens and closes based upon differences between the pressure at the outlet passage and the pressure at the upstream inlet passage. 6. The cooling fluid pump according to claim 5, wherein the check valve comprises a ball and a ball seat. 7. The cooling fluid pump according to claim 2, wherein the impeller has a circumferential upper end and a circumferential lower end and wherein the drain inlet opening is vertically located at or below the circumferential lower end. 8. The cooling fluid pump according to claim 2, wherein the drain passage is made of a different material than the pump chamber. 9. The cooling fluid pump according to claim 2, wherein the upstream inlet passage comprises an inlet opening; wherein the downstream outlet passage comprises an outlet opening; and wherein the inlet opening is vertically located lower than the outlet opening with respect to the pump chamber, and wherein the upstream inlet passage slopes downwardly away from the pump so as to drain cooling fluid from the drain outlet opening. 10. The cooling fluid pump according to claim 9, comprising a recess in the upstream inlet passage, wherein the drain outlet opening is located in the recess. 11. A cooling fluid pump for cooling a marine engine, the cooling fluid pump comprising a body having a pump chamber in which an impeller is driven into rotation by an impeller shaft to pump cooling fluid from an upstream inlet passage in the body supplying cooling fluid to the pump chamber to a downstream outlet passage in the body discharging cooling fluid from the pump chamber; and a drain passage in the body, wherein the drain passage connects the pump chamber to the upstream inlet passage for draining cooling fluid that settles by gravity in the pump chamber back to the upstream inlet passage. 12. The cooling fluid pump according to claim 11, wherein the drain passage has an drain inlet opening that receives the cooling fluid and a drain outlet opening that discharges the cooling fluid, and wherein the drain inlet opening is located higher than the drain outlet opening. 13. The cooling fluid pump according to claim 12, wherein the drain inlet opening is facing downstream with respect to the pump chamber. 14. A cooling fluid pump for cooling a marine engine, the cooling fluid pump comprising a pump chamber in which an impeller is driven into rotation by an impeller shaft to pump cooling fluid from an upstream inlet passage supplying cooling fluid to the pump chamber to a downstream outlet passage discharging cooling fluid from the pump chamber; and a drain passage that connects the pump chamber to the upstream inlet passage for draining cooling fluid that settles by gravity in the pump chamber back to the upstream inlet passage, wherein the upstream inlet passage comprises an inlet volute having a recess formed therein, wherein the drain outlet opening is located in the recess of the inlet volute. 15. A cooling fluid pump for cooling a marine engine, the cooling fluid ump comprising a pump chamber in which an impeller is driven into rotation by an impeller shaft to pump cooling fluid from an upstream inlet passage supplying cooling fluid to the pump chamber to a downstream outlet passage discharging cooling fluid from the pump chamber; and a drain passage that connects the pump chamber to the upstream inlet passage for draining cooling fluid that settles by gravity in the pump chamber back to the upstream inlet passage, wherein the impeller has a circumferential upper end and a circumferential lower end and wherein the inlet opening is vertically located at or below the circumferential lower end so as to drain cooling fluid to a level that is below the impeller. 16. A cooling fluid pump for cooling a marine engine, the cooling fluid pump comprising a pump chamber in which an impeller is driven into rotation by an impeller shaft to pump cooling fluid from an upstream inlet passage supplying cooling fluid to the pump chamber to a downstream outlet passage discharging cooling fluid from the pump chamber; and a drain passage that connects the pump chamber to the upstream inlet passage for draining cooling fluid that settles by gravity in the pump chamber back to the upstream inlet passage, wherein the upstream inlet passage comprises an inlet volute having a wide inlet end that tapers into a narrow outlet end and wherein the downstream outlet passage comprises an outlet volute having a narrow inlet end that tapers into a wide outlet end; and wherein the drain passage connects the narrow inlet end of the outlet volute to the wide inlet end of the inlet volute. 17. The cooling fluid pump according to claim 16, wherein the downstream outlet passage is one of two outlet passages that are oppositely oriented with respect to the impeller. 18. A cooling fluid pump for cooling a marine engine, the cooling fluid pump comprising an upstream inlet passage having an inlet volute that supplies cooling fluid to a pump chamber in which an impeller is driven into rotation by an impeller shaft to pump cooling fluid to a downstream outlet passage having an outlet volute that discharges the cooling fluid; wherein the inlet volute and outlet volute are stacked with respect to each other along an axis along which the impeller shaft extends; and a drain passage that extends from an upstream end of the outlet volute to a downstream end of the inlet volute and is sloped so as to drain cooling fluid that settles by gravity in the pump chamber back to the upstream inlet passage. 19. The cooling fluid pump according to claim 18, wherein the drain passage has an drain inlet opening receiving the cooling fluid and a drain outlet opening discharging the cooling fluid, and wherein the drain inlet opening is located vertically higher than the drain outlet opening; and wherein the drain inlet opening is facing downstream. 20. The cooling fluid pump according to claim 19, wherein operation of the impeller increases pressure at the downstream outlet passage and decreases pressure at the upstream inlet passage; and further comprising a check valve that prevents cooling fluid from draining back to the upstream inlet passage through the drain passage during said operation of the impeller.