If you're just looking for fun party speakers, the UE MegaBoom is fine for that. If you get 2, you can sync them so they play in stereo. Good battery life, and supposed to be weatherproof so they should handle beach usage fine. I use the regular Boom as an outdoor pool speaker and it does a decent job for that, so the larger one should be fine.
That being said, you are dealing with the normal limitations of wireless, portable speakers. For parties where you're just trying to crank some background tunes, they're perfect. If you're planning on doing any serious critical listening, well, they are what they are.