when you are at the beath ,the most important is to keep your eyeglasses away from the sands .Do not put the eyeglass frame under the sunshine for a long time. Never lay your glasses face down. Keeping the lens surface face up prevents them from coming in contact with things that can scratch the lenses.Store your eyeglasses in their case when you're not using them. It keeps dust and particles off and prevents them from being inadvertently crushed or damaged if something falls or gets dropped on them.
To take good care of your eyeglasses at the beach, you shall keep your eyeglasses away from sands and avoid long time expose to sun. So, you shall always take a glasses case so as to keep eyeglasses while you don't need it. Please do not put your eyeglasses on the beach casually or put it in your hand bag without case, or put the glasses on your head. that actions will hurt your eyeglasses lenses. So, you shall put them in special eyeglasses cases, and then put it in your handbag or put it in some cool places.
What do you mean by taking care of eyeglasses at the beach? Most of us wear a pair of sunglasses to protect their eyes from the UV rays when they are playing at the beach. They don't need to take off them. So they don't need to take care of them as long as you won't let them fall down from your nose bridge.
Dude, do you mean how to clean your eyeglasses from getting scratched by sands or damaged by sea water? If you do, you can rinse your eyeglasses with clean water and then wipe the water with a piece of clean soft cloth. Don't wipe the sands directly by the cloth and don't dip your eyeglasses into the sea water.
Rinse your eyeglasses with clean water if you get some dirty on them. If they are hard to remove, you can turn to an optician immediately. But I have never met difficulties in taking care of my eyeglasses when I was playing at the beach.