Another vote for holders like Croakies that hold your glasses tight to your head. At Blizzard Beach, I had just regular glasses holders on my glasses (not Croakies, sad to say). I fell off my inner tube while going down the Lazy River. As I went under, the glasses strap had enough boyancy to float up off my head. As the string got wet, it got heavier and sunk, so my glasses sunk to the bottom.
Lesson learned: when in water, use Croakies.