Re: where did you guys mount your amp on your boat for the stero?
At my local sandbars, there is one that more or less is the party sandbar. People set up generators with DJ systems/speakers. A boat system is the quietest thing around! If you want peace and quiet there are plenty of other sandbars...
Anyway, with that out of the way, I have a 12" sub mounted facing down, right above the passengers knees. The amp is mounted on the transom, to the very edge on the starboard side. (it isn't the transom exactly, there is a false wall between the actual transom and the interior, its mounted to that.) No heat issues, there is lots of air space in the engine compartment, and its only a couple feet from the battery.
Which brings up the bigger issue.... Keep in mind you have to double your distance that you are running cables to compare it to car audio, since you have to run both power and ground. 15 feet of power + 15 feet of ground means you have to have sized the cable for 30 feet, not 15 like a car. A decent amp at 30 feet takes some serious cable! Its easier/cheaper to run long signal and speaker wires than power.
Finally... as far as main speakers, you can only push a small speaker so much. Add more speakers if you want it to sound better at high volume, don't try to push your overworked existing ones more.