Have you considered the Exile Javelin amp?
It's a true 5 channel amp that's seems like a good fir for your application. I'm in the middle of my own audio build and I got a Javelin to power my cabin + sub. I've heard this amp on several of my friends' boats and it really jams.
The four cabin channels EACH give 100 watts at 4 ohms -or- 150 watts at 2 ohms.
The 5th channel (sub) gives 500 watts at 4 ohms -or- 800 watts at 2 ohms.
When I un boxed it last week there was a CAE certification included showing how my actual amp tested out.