Hi
A couple of issues occur to me if I were to circulate the water in the hot water storage tank rather than coolant through the coil. The most obvious is that the system would have to be drained (and therefore not functional) if the boat ever goes to a cold climate. Well, I don't know how much hot water it would produce in a cold climate anyway, so that may not be a problem.
If I were to stay long term in a cold climate again, I would install an oil furnace with a water heater coil and also some heat radiators around the boat. I didn't have a heat coil in the furnace on my old boat, but I helped install a couple of systems like that back in Norway. In that case, the hot coolant flowing through the engine block was very welcome to keep the raw water side of the engine from freezing.
As for the oil coolers. They look a bit small to me. I would probably need four and then we're over $200 by the time I have them on the island. I did think of using two car coolant radiators in the box, but don't know how much it would matter that the fins are not at 90 degrees to the sun. (If I use the double greenhouse glass, the collector itself isn't really visible, so the "not pretty" factor of two old car radiators would probably not be an issue.)
It would be very rare that the sun shines through the collector fins, but even so, I wonder whether it would collect less than a normal collector. I mean, the energy gets into the box and hits the black metal, the box is insulated so most of the heat energy stays in the box. Does it matter which angle the rays hit the metal at as long as the box itself is at a good angle to the sun??
Owen