I have an '89 Johnson 70. It is always rinsed with fresh water immediately after retrieval from salt water. I never crank the motor w/o water supply. What is the recommended service interval under these circumstances? Do older impellers fail catastrophically or simply lose their flexibility and become less efficient? I've got 3 1/2 years and about 300 hours on my current waterpump(complete kit), 80% in fresh water. Should thermostats be replaced along with waterpumps? Thanks, Pete