400w is more than enough for me. I'm no expert though so please don't take that to heart!
Should have mentioned im running a sports cruiser (Member because I used to have a RIB) so I have her installed in the cabin at the end of the bed (closest to the electrics but plenty of air)
If I was installing in a RIB, i'd use STRONG STRONG glue to glue a timber frame to the inside, high as possible part of the console, and screw the amp onto that. In a RIB, the higher in a console the better IMHO, unless your console is totally waterproof