Here are a couple of quick questions, and some ideas to start.
First, is the stage protected and secure? Are your going to have to tear it down every night? Either way, be sure that you get a rack with locks for your mixer and effects. Also get a DJ style CD player for the rack. I'd get a Mackie CFX (12 or 16) if I was spending other people's money. The biggest problem with outdoors is that the bass disappears into the sky. You should consider sub-woofers and put them as low as you can. For the PA, I'd get 3 pairs of full-range speakers, one pair points in toward the dance area, the next pair points out further down the beach, and the last pair point back at the band, as their monitors. (As described in my columns)
Fender and JBL make some nice all-weather cabinets. You're going to need a good amount of power, figure 200W for every speaker, plus the sub-woofer. Plus, even though the bands are small, you're going to need a lot of mics, because you're going to have to mike the drums and guitars. But buy cheap ones ($50 max). You'll need a snake and cables - can you put the mix board up on a lifeguard tower (too cool!)?
OK - reality check - do you really want to “light-up” the entire beach, or just the dance area in front of the band? It gets a lot simpler when you don't have to cover the large area.
Do you have a local music store that you trust? You're going to need one, and you're talking about some serious bucks.