A 22" Hydromite , Trinity 15turn and an Octura x432(balanced and sharpened) should be a fun and fairly fast package on 7 cells.
As far as making your speedcontrol a little more water resistant use shrinkwrap. Leave about 3/8th's of an inch hanging past the ends and fill with silocone or epoxy. The best alternative would be a speed control from RCHydros available at OffshoreElectrics.com.
To play it safe don't let the silicone or epoxy touch the board.