Bob,
Unless you really over-prop the thing, the 6 volt Pittman won't
generate much heat in a modern ESC. Water cooling is for the high
amp stuff. At 3:1 gearing, you should be able to turn a 2 1/2" prop easy.

With a whip antenna topsides, I can't see you getting any noise
to the RX, but it's possible I suppose. It would be nice if the
bigger gear was derlin/nylon, but try it out, it'll probably be fine.

I always stuck to direct drive for simplicity, so I have no experience
with geartrains. Some of my pals used them, and they were OK.
(My buddy Dave had a 1:1 "Z" drive in his Sea Maid to keep the
motor in the proper place.)
Dave