I can't provide specifics for the boat but can give some general instuctions based on automotive belts.  My engine still has the old style v-belts.
There should be a belt tensioner that has a square pocket for a 1/2 inch drive ratchet.  Using the ratchet to relieve the tension, slip the belt off and let the tensioner back into position.  The tricky part is remembering the belt routing.  Taking a picture and having a print handy might help.  Most cars now have a sticker with the routing to help.  
I don't know about the sea water pump that is crank driven on the new engines.
					
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Terry
Current: 07 Crownline 255 CCR cuddy - 350 Mag MPI/B3 "Casi Cielo"
Previous:97 245 Sundowner 5.7GI/SX 
Tow vehicle: 1986 GMC 2500 Camper Special 454/TH400
