After 3 years of owning my boat and sweating it out I had my Timing Belt and Waterpump replaced at the beginning of this season. The Waterpump is not marinized and the entire kit was ordered with OEM parts for a Lexus LS400, GS400, or SC400 right from the dealership.
My good friend is the GM at the local Lexus dealership and he linked me up with one of their Lexus mechanics. He has done 150+ car timing belts in his career. He was a bit hesitant, but did some research on this site and tackled it with absolutely no issues. He did it in the dealership's parking lot on a Saturday. His feedback was that it was no different than a car, with the added bonus of more room to work. He didn't even take the ski pylon off.
$260 in parts and $250 in labor and I can breathe easy for the next 10 years. I have a 1999 with 420 hours and the belt was never replaced, and honestly showed little to no signs of wear or Waterpump leakage. But $510 was well worth the peace of mind.
Best of luck, call the Lexus dealerships in your area and see if you can get a mechanic to moonlight it.