So, since Cal's made the point that cheaper amps are coolio....
check out the Kenwood 7201 -
http://www.epinions.com/pr-Kenwood_KAC-7201_Car_Audio_Amplifier?zip=92882&x=5&y=12Cheap Internet price $130 delivered, usually around $150, but seems like a hell of an amp for the money (even though I don't pretend to know anything about stereo stuff).
460W RMS into a 4-ohm sub, or
230W RMS x 2 into 2-ohm loads, or
150W RMS x 2 into 4-ohm loads.
Built-in crossovers, line or RCA with preouts and a little fan.
I originally wanted to bridge one for a sub, one as a 2-channel for the four boat speaks and a third to drive the tower speaks.
BTW, the v-drives have the battery mounted under the stbd aft seat cushion. One play I noticed for a second or third battery would be to go into the cubby and turn left - place them in-line, running fore and aft, with the seat back on one side and the hull side on the other. Not sure how the seat is on the ddrives.
On the driver side, I think there's room between the bow seat back and the hull wall so long as you slide them in from the driver's feet area and run them in-line fore and aft. Might only be room for one on the driver's side.