Who wants to be worried about losing glasses while you are having fun? Get them adjusted so the fit is snug, but for water parks and some coasters, I'd get a CROAKIE just to be sure.
My DH is on the verge of being legally blind w/o glasses. There is no way he would enjoy rides or water parks without them; he literally cannot see his hand in front of his face without his glasses.
The answer for security is a CROAKIE. Not an "eyeglass string" that has little loops that go around the earpieces at the temple; those are not good enough for active pursuits. A Croakie is a tube made of neoprene rubber, and it is short. You insert the earpieces of the glasses into the ends of the croakie, put the glasses on, and then slide the croakie ends further toward your face until the glasses are tight on your head. A croakie will hold them on your face even if you hit water head-on. (And if you don't have it tight enough, and your glasses do come off, they will float; the croakie is rubber.)
Croakies are available all over the place in Florida, even at drugstore counters. Further north, look at a sporting goods store.
They cost about $6; well worth it for peace of mind.