I think it's a personal pref. I prefer 4-strokes on any plane 60 size and above, of course, I don't fly extreme speed planes either.

As far as seaplanes go, I use a 4s on my goldberg cub, because I fly it on land with a 4s,

As for other planes, (Seamaster, Mariner, etc) I prefer 2 strokes only because I don't want to run the risk of ruining a good 4s by dunking it (Or worse, having it sink!)

My dad, on the other hand uses nothing but 4s in everything including seaplanes.