You can install them yourself. To take them off there is a little retainer clip that holds the shock on the ball socket(there is one on both ends). Just stick a small flat bladed screwdriver underneath the clip and pry out (approx. 1/8") and pop the shock off the ball. Take the shock into an automotive store and they can match up the length and strength of shock that you need. Will be a lot cheaper than a boat dealer would charge for the shock.
Agreed, they are very simple to replace. I tried to source mine from the auto parts stores, went to three, and they don't stock more "generic" sizes. I ordered mine online.
Some sites have instructions for matching up what you currently have. I actually increased my strength as the originals collapsed when the carpet was wet.