Re: speaker placement
I assume you plan to mount the speakers under the seats. My boat (24-foot 'toon) came with 4 6.5" round speakers that seem to do a good job. One at the captains seat, one in the back bench seat opposite the captains chair, and two up-front in the middle of the bench seats. However, typical boat placement like this (to me) makes it tough to get lots of sound out of them without a lot of wattage. The speakers are low and face downward, and since higher frequencies are directional, this isn't exactly ideal. But in my case, there's really nowhere else to place them. If you want to listen while cruising with engine noise and air rushing by, I'd definitely look at a separate amp from the head unit to give it some extra ooomph. The guy who owned the boat before me put a Sony head unit in to give it some more power, but even at 10-15 mph a lot of the higher frequencies are drowned out by the wind.
Also, one thing I'd like to do is add a subwoofer to my system. Not so I can go down the lake rattling windows, mind you. A sub REALLY fills out the sound, and I think it's needed especially on boats where the space isn't small and confined like cars. I've looked into them a *little*, and saw a pioneer marine sub/amp combo that got really good reviews from customers. The sub is free air, so you can mount it anywhere--no need for a box, etc.
One last note: Since a boat is open, it's not like a car where the speakers sound is "combined" (best way to put it?). When you're sitting on the front left bench seat, that's the speaker you hear. Thus, if I were you, I'd use the same speaker (6x9 or whatever) at each location.
