Hey Sue,
The first time I snorkeled my FEV1 was 40%. During my first time, I was laying on a raft and would put my head down in the water to see periodically. We did off beach snorkeling to start.
On most arranged snorkeling excursions the operators will give you a bouyancy vest. While this is decent for helping you float, I found it to be restrictive to my breathing and made me fee claustrophobic.
Now I snorkel and swim wearing a bouyancy belt. It is basically a piece of dense foam that you wear around your waist. When wearing it you can easily flip from a position of sitting in the water to being face down. I can even be in some fairly choppy surf and not feel to horribly stressed. Search for flotation belt or bouyancy belt and you should find lots of options.
I love the belt because you sit higher in the water than with a bouyancy vest. Also, it is really easy to pop up for a second, take a rest, and/or clear your snorkel.
I say give it a shot and have a strong swimmer with you holding your hand while you explore the beauty of the sea. My hubby (he is a certified diver which helps) and I have snorkeled the Great Barrier Reef, many sites in the Bahamas, Bora Bora, FL, etc. Better to try now than to never have tried I say <img src="i/expressions/face-icon-small-smile.gif" border="0">
Happy floating!
Steph