I don't think running synthetic trans fluid is going to be any advantage in WB boats.
In a truck pulling the boat to the lake, maybe, because it is much harder to give off heat through an oil to air trans cooler.
With a boat, however, you have a virtually unlimited supply of cool water (whats the warmest your lake ever got?) that is run through the trans cooler. Plus, there is no TC so none of the slippage that occurs with shifting gears and TC slippage which is what really heats things up. So when you change the trans fluid at 100 hours, its probably not because its broken down...its more because its dirty.
For the same reason, I would not run synthetic trans fluid longer than the reccommended interval for dino.
I do run synthetic in both my boat and truck engine. I can be swayed that heat is much more of a factor in that respect.