Yeah, for the distance stuff like Tybee I think you need the type I vest (capable of floating your sorry a$$ upright if unconscious).
I have the Magic Marine vest and it's nice & comfortable with good range of motion on the shoulders. It's not type I, but works for all but the worst conditions.
One thing you may want to consider is how fast you can swim with all the crap you add to your vest and harness. Cats can move pretty quick on their sides, and if you can't swim too well to begin with, imagine how slow you'll swim with your spraysuit, vest, camelback (with 8 lbs of water), GPS, radio, etc. If you don't have a type I vest, you may not even be bouyant with all that....
