My 2 Kawasaki STX skies aren't supercharged but they are 4 cylinder 4 strokes. I just run Mobil 1 Euro 0-40 in them. Your manual should tell you that several different weights of oil is acceptable depending on temp.
Some of the new ones have cats on them so pay attention to oil selection if that bothers you as some of the marine oils have higher levels of zinc because most boat engine don't have cats but live at high RPMs most of the time.
Funny you mention that, I seriously thought about that as I used to run that in my Porsche. I even thought about 15W-50 M1. Just not sure if the "Marine" spec really means anything. Does it? Does it really resist corrosion etc better than a M1 or similar? 5qt jug of M1 from Walmart is pretty hard to beat cost wise. I'm certainly not opposed to Amsoil as I really think it's great oil, just don't have a need to waste money however.