Some thoughts. As I didn't plan on getting myself wet or anything, the first thing that popped to mind was a pretty summer dress in bright colors like these two from Free People shown here. Just imagine what a pretty beach pic these dresses would make! And the thin material and relatively cheap price is perfect for the beach, as you wouldn't have to constantly think about ruining it. 
Ultimately though I ended up wearing cropped jeans as the practical side of me pops up, with the fear that skirts may be blown by the wind and it could get messy. You always want to wear pants when things get messy. Of course, shorts work too, but I was also scared of mosquitos too since it was night- I know I think too much! (Both from Forever 21)
Topwise, I suggest a pretty floral top that would also make a pretty picture (left: UO, right: Forver 21).
And if you're worried about a tan line (as I'd be if I'd went in the afternoon), tubes are a great option like these two from Forever 21 (left) and UO(right). I especially love the white tube, because its so simple but elegant and fun.
Then came shoes. These are the times when sandals/flip flops become your best friend, as heels will sink in and are impractical and sneakers are too hot. Remember to get super cheap ones, as you wouldn't want to ruin your fancy pair on the rough sandy shores of a beach (above from UO). I bought a pair of flip flops for less than $3 especially for the occasion! 
Then came a bag to put all your stuff in. At that moment, I wished I had my white Le Sportsac huge tote with me. But alas it is in Chicago. But really, just stuff everything (mat, sunscreen, food, water, shades, umbrella etc) into a big beach bag like the one above from UO. And just in case you want to head over to the bar for a while and don't want to bring everything with you, consider bringining a small clutch like this one also from UO to put your valuables in. Leaving valuables anywhere is not a good idea. Plus, it'll look more chic then holding your wallet around. And lastly, don't forget to bring a cardigan. It CAN get cold at night on a beach- think OC beach parties with everyone putting on college sweat shirts and cardis. And that is my whole thought process prepping for this beach party. I know, I think too much. LOL. Hope everyone has a fun weekend as well!
