Get an ST-5 from one of the box stores or your friendly plumbing supply house. Locate it near the cold supply pressure line, e.g. DOWNstream of the pump. Anywhere on the cold line, it does not need to be close to the pump, can be at the other end of the boat! Pick a location unexposed to seawater, the tank is steel, but has a bladder. Fasten it securely with some aluminum or stainless bands, it can be suspended or mounted in any manner wherein it will be well-supported, but not by the pipe fitting. Shut off the water, cut a tee into the cold line, pipe the branch of the tee to the tank. Set the air pressure in the tank about 2-3 psi below the cut-in pressure of the pump. Re-charge the water line. Leak check. Done.
Size isn't critical, the ST-5 is proven, it's pressure rated higher than your marine system requires, but that's a non-issue. It's cheaper than a marine "accumulator" and performs the same function.
I have a variable speed pump which does not require an accumulator, but I still use the ST-5 to help with hot water expansion, and just to keep the pump from cycling as much. Works great.
An aside that if you want the tank function as a thermal expansion absorption device as well... - if your water heater has a check on the inlet, disassemble the check valve and make a small notch in the valve seat. Just enough to keep it from sealing 100% This small bleed will allow enough backflow to prevent thermal expansion overpressure, while still allowing the check valve to perform its intended hot water backflow function. So your tank can perform dual functions.
Same here but I bought the 2 gallon size. 4-5 years later it's still going strong. 
