I could be wrong here but as far as I know, the plate coolers that are found in the bell housings for trans cooling are also water cooled and not air cooled. I just don't think that air cooling on a boat makes much sense. They spend too much time at slow to stopped speeds and with a constant supply of cool water being expelled after use out of dumps or exhaust even while sitting still why bother? Just my 2ct's worth.

You are correct. The aftermarket bellhousings have dual coolers (one in each side). One is used for the trans and one is used for engine oil.
Eddie

Thought I would post pictures of the bell housing and where the plate coolers go since I gave some incorrect information in a post - was the least I could do. in some automotive applications plate coolers can be air cooled, but as a general rule and as mentioned above are water cooled in marine apps. The left is the Hardin Marine version. The second is the one we are offering. The hardin marine is pretty neat in that you can place a PS cooler in between the two plate coolers at the crossover and still run one water circuit to cool all 3 (if of course you haven't overused your water's cooling energy at that point).