A second battery is a good way to don't stay stuck on the lake.

Go for a 4 channel class-D car amplifier, they draw alot less current than classical class A-B.
I run a very little Soundstream 5 channel PN5.640D to power the whole boat (4 speakers onboard, 4 speakers on the tower, 1 subwoofer). We can spend a day on water without emptying the battery, thanks to class-D