I don't think jim is wrong.... Engine size all depends on the type of plane your flying off floats.. In most part all the float planes I have, have the recommended power.. And in most part its at the bottom of the scale.... A .40 size cub will fly off water with a 40 4-stroke... Another factor is your flying ability, If your not that experienced then you may need that 50 for a dumb thumb....
Nobody is wrong it's just that jbflier needed to give us more info on what he wants to do with the aircraft and if he is a beginner in r/c, a novice.??..etc...