I personally own and use the DiCaPac waterproof case and have used it in and around the water at the beach to great success. I also take it caving regularly after destroying two cameras. My main complaint is that the finger holes are a bit awkward and stiff and make zooming in and out a bit of a pain. Additionally, not all (but most) lenses fit. You can get em for around $80-90 bucks on eBay from memory so its a worthwhile investment if you plan on doing a lot of shooting. They're also supposed to be waterproof to 5+ meters I think (I've never taken mine that deep though so i dunno about those claims...)

You could get something like these rain-sleeves which are a cheap inexpensive way of protecting your camera against most water and sand however they will not protect the front element like the DiCAPac's and EWA marine housings. That being said, in conjunction with a UV filter sand shouldn't be an issue, just water (via the front of the lens) and if you stay alert and out of the actual water you should be fine.

The super expensive option is also getting a full diving housing which usually cost more than your camera and have the added bonus of requiring individual 'ports' for each lens you have. I personally have one of these housings for a smaller point and shoot (canon Ixus 220 HS) and I use that for deeper diving and underwater video (looks beautiful!)

Hope that helps!