If you are maintaining a trailer boat, this is an easy proposition. At least once a year, remove the raw water pump impeller. Give it a squeeze to see if the vanes are hardening. If the vanes have lost their elasticity, replace the impeller. For reusable impellers, grease the vanes lightly with petroleum jelly and replace.
Some boaters replace the impeller with a new one each season.
On boats equipped with a heat exchanger, additional cooling system maintenance is required. The freshwater cooling system is similar to those used in automobiles and should be tested to ensure the proper level of coolant protection is maintained. Use any commonly available tester. If coolant protection is inadequate or the coolant is several years old, the system should be drained, flushed, and refilled with fresh coolant. The coolant, or antifreeze, does not wear out quickly, but it does wear out.