Sorry - I was thinking about my boat when I answered your question. For an 85 S&S I'm not sure I'd want to spend the $2k to put the indirect cooling in a boat of that age.
But I still stick with my recommendation - If you'd enjoy the boat more and boat more frequently, then I would not hesitate launching into salt water.You bought the boat to use it! Just make sure you go through the cleanout procedure every time - particularly an engine flush. Take comfort that you have a fibreglass boat, stainless fittings and a bronze prop.
Still not sure what to do about the trailer.
2012 X-14V, 5.7l Ilmor MV8, A plate, pro package w/ ZFT-2, heaters, stock everything else.