Check out a “wind up” radio, I have seem them and work well without the need of a/c or batteries. Check out the link below:

Get a battery charger that work off the car lighter and then you can charge it on the way to the beach.

i dont know if they have them but its a portable generator that u turn the crank and it makes electricity

Honda makes very small generators which perhaps make the least amount of noise of any on the market. Get a long extension cord to keep the noise far away.

i think Radio Shack has something like that – give them a try

A generator will work, however the noise of the generator may ruin the fun for yourself or others at a beach. The batteries in a radio should run for several days without problem. The easiest thing would be to make sure the batteries are fairly new when you go to the beach for uninterupted music pleasure. Or, you could get one of the camping or emergency radios that have a small generator inside and you just wind it up to play it. No engine noise from this generator.