Re: Changing impeller (costs)
list price on a water pump kit for your engine (p/n FK1204) is $65. You should be able to find it here on i-boats or on ebay for around $50.
I always do this kind of work myself for quality reasons. The local marina (southwest florida) does not like force engines and i think that they give them to their least competent mechanics. Which causes their hours to be higher and their quality lower.
I do my own annual service at an average cost of $50 per year per engine (including occasional starters and batteries). My annual service is to change impellers, change gear oil, replace gearcase seals (every 2 years), replace fuel pump diaphrams (every 2 years), synchronize carbs, adjust idle/linkage, new spark plugs, check anode, check battery cables. It takes one day to do both engines ('88 85HP).