Once you get to where the water is over your head it really doesn't matter how deep it is, 10 feet might as well be 1,000 feet. If you can accept this it makes it alot easier to do things like scuba diving. If you can float on your back and tread water you should have no problems snorkeling or diving. And if you can't float on your back or tread water then you have never been taught how to properly.
And besides there are very few critters in the water that will try to eat you, and most of the times they do try it is by accident where they mistake you for something else.