Regarding jelly fish:
I know this will sound silly, but it works.
Wear long pants (jeans work best because they are thick enough) and a long sleeve t-shirt and aqua shoes or old sneakers.
Issue: The clothes will weigh me down. This is false. Clothes float as do you. Also, you should be wearing a life vest if you are just snorkling.
Also, there is no better sun protection that jeans. They have an SPF rating of 1000. I am allergic to aloe (therefore, most sun protection cream) and normally have to swim in a dry suit or jeans/khaksi and a shirt. Dry and wet suits are expensive. Jeans and shirt are not.
Wearing my clothes, I have never been stung.
My wife and I are going on a cruise 4/5/2007, and for snorkling, we are packing long pants for both sun and jelly fish protection.