Well, here's my 2 cents worth (maybe overpriced). I race boats and cars. The powerglide can be made near bulletproof, is lightweight but you only have 2 gears (probably all you need). I run 190 MPH in 7 seconds with one in a Blown Alcohol motor in a 48 Fiat. This would be my choice for a boat. Low gear ratio of 1.76 to 1 is stock (most..some are 1.82) Turbo 350 or 400 gives you 3 gears..but with over 200 ratio on the low, cannot see how low gear would be of use. Guys used to run Turbo 400 because they were so strong, but I feel the powerglide can now be made just as strong..maybe stronger, with less weight and cheaper too. The boat I have is a Blown alcohol Flat and I don't use a transmission. But if I were going to shift it, I would build a powerglide.