If your strategy is to gear and prop your boat for top end and have a lower gear for takeoff, the powerglide won't work because 1st gear is too low! If you want to shift it you'll use 2nd and 3rd of a 3 speed tranny. Which one? As stated, the t-400 eats more of your power but is MUCH stronger (and heavier!) Water isn't as harsh a running environment for the drivetrain as pavement is except for the hopping and re-entry. I've used t-350's on 2 boats with mild big-blocks and 22% overdrive without problems. With bigger horsepower and overdrive numbers, I'd choose the t-400