The Aerobie Sharpshooter line floats in water, not the Epic or the Arrow, just #1, #2 and #3 sharpshooters.
Lightning discs float in water only if they say they float in water, usually printed right on the hotstamp, only certain models float, others do not so watch carefully when choosing lightning for floaters.
And, of course, Innova makes 2 that float. The Dragon is the most popular driver for the lake filled courses in the area and the hydra for putting at those baskets near the waters edge.
Now some of the lighter R-Pro from Innova is being reported as floating, I have not tested this.
Around here, just because the disc floats does not mean you will get it back...sometimes it just means you get into a stare off with the local gator, and he seems to win every time.