Re: $3000/mo cruising budget
Bladder? Bladders aren't good conductors. What you want is, after all, a normal "radiator". Thta's old tech too, folks used to run cold water through yards of copper pipe as a cooler, pre-air-conditioning. Again, it works fine with fresh water but your salt water pump may clog, and critters will grow inside whatever (pipe, bladder) you are using for the heat exchanger. Aside from monel pipe being unaffordable these days and copper being almost as pricey...If you drop an intake pipe below the thermocline (10-20 feet typically) and filter the intake water, you should be able to get good cooling, until you have some kind of scaling or clogging. Might even be able to get acceptable results with the cheaper PEX tubing that is used for in-floor heating and cooling as well.
Only one way to find out, right? (G)