I can tell you my past beach experience and what I plan to do in the next month, where I will be around the beach for a month. And offer some suggestions.
If you're like me, and you want to use the Watch for more than exercise, then your iPhone will be nearby. 30 feet. So protecting the iPhone will go hand in hand with care of the watch.
My last day at the beach was mostly sitting in a beach chair. No problem just wearing the watch, phone inside my back pack. If I went into the water, watch goes off the wrist and into a hard eyeglass case, with a cloth to prevent movement inside. But normally someone stayed behind at the umbrella. Just as I would do for any watch or valuables.
Being that you will be in Mexico, you may not want to leave it unattended. So leave in in the hotel safe, unless you are in possession, like running with it.
With care, sand should be no problem, but be aware of it. Sand will scratch glass and metal. As people said, it's sandpaper, BUT you do need to apply some pressure, like sandpaper. So if you get some, blow or wipe the sand off gently.
One trick I do, is to wear an elastic tennis sweat band covering the watch. It hides it from eyes and sand, dirt. No sand should get underneath, but take care.
With precaution, you should be fine, the same precaution you might take with your eyeglasses, iPhone screen or wallet. Do rinse in fresh water and dry. It removes any salt spray and dirt. Pat dry. But if you plan doing all day water sports,leave it in the hotel.
But do take it! Many hours will be spent outside the beach, so have fun with it.
Enjoy your trip!
