The biggest issue I think you'd have in a catamaran is mounting the battery. The smallest ones that will do the job are going to be 10 lbs and its easy to get to 40 lbs. Where are you going to mount it? Think of the slamming a cat takes and the potential for turning over (or turtle). The battery mount has to take the jolts in all directions. Can you get inside the pontoons? That might be a good spot, and you could also mount the speakers from the inside. Watch where you put the weight - a battery makes a big difference - both front and back and side to side.
If you can't get the battery figured out, buy a waterproof VHF hand-held and store it a small winch-crank pouch near or on the mast.
Solar chargers work, but the inexpensive ones are slow and the fast ones are expensive. You have mounting issues with them too. There are flexible forms, but its not the kind of think you can walk on.
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