I live on the coast of Maine, and we don't have too much sand (the glaciers took it all south to make Cape Cod), but I do get near a lot of salt water. So here's some ocean/beach gear tips that have served me well...
The thing about beaches is that it's not just the sand, but the salty air. So if the wind is blowing, cut a hole in a plastic bag, put your camera in the bag (get one just big enough for your camera) and push the lens out through the hole (letting the plastic stretch around it). Now cut a smaller hole in the back for your viewfinder. A strip of tape on the bottom to seal it up, and you've got everything protected except for the lens barrel. With digital, you don't even have to open it up to change the film. (These bags are also great for when it rains.)
If the wind and surf isn't up, simply use masking tape to cover the bottom of the camera, over the flash card door, and the flash terminal (if you're not going to use it). Anywhere salt and sand can get in. When you get back to your room, simply pull off the tape and wash the camera and lens barrel with a damp cloth.
Next, don't take your camera bag to the beach - even outdoor bags, like my Lowepro Orion AW, aren't designed for it, and if they get sandy or salty, the sand and salt will always be in there.
Instead, use a regular Playmate cooler. They're white on top, so they don't soak up the heat, you can pour a bucket of water over them and your gear stays dry, they're quick to open and lock shut (even with one hand), and to thieves they look like a cooler of beer.
I've lined mine with foam rubber on the bottom, and then more foam, zipped up tight in ziploc sandwich bags, taped to the sides and customized to fit my camera and an extra lens. (Since it's all in plastic, you can easily wash it, but you won't really need to if you keep it shut.) The hard case and soft inside is also great if your gear should take a fall or a hard knock. Toss in a small towel on top, a couple garbage bags tucked in the sides, and you're all set.
Finally, for your question of changing lenses, if you do it inside the cooler, and quickly, with your back towards any wind there is, you won't have the trouble of salty wind and sand. So yeah, just use common sense and you'll be fine.