Ditto the comments about replacing the pump, I just replaced the water pump in my '93 40HP Mariner Magnum (with great input from ml2trick, thanks again, BTW!), so now I feel like an "expert", lol.
In my case, the dealer I bought the boat from six years ago said they'd replaced the impeller when they took it in on trade, and it performed well up until this spring. When I threw on the earmuffs and fired it up in the driveway this year, the water was trickling out of the tell-tale hole and was too hot to the touch (@ 130 degrees). Since I hadn't addressed the water pump since I bought the boat, I figured, rather than replacing the impeller only, which would run @ $20, I'd be better off getting the whole pump housing kit (@ $75), which included gaskets as well. That way, I knew I'd have a definitive baseline of exactly what was replaced when.
Since this was my first go at pulling a LU, it took me a bit longer than it normally should, but overall, not too tough a fix, and now that it's done, the pump is running well and there's a robust stream of water pumping out of the tell-tale hole.
Do yourself a favor, as these other guys have posted, don't take a chance on damaging the power head by going cheap, in the long run a full kit replacement is insurance against potentially serious damage to the motor.
When you figure the cost of replacing a 20HP motor, the cost of a pump kit is a relative bargain!