After reading this thread and watching every online video I could find on the topic, I'm going to try the swimming pool noodle approach - essentially adding a pair of removable foam pontoons to the landing rails. I'm going to use the following video which looks about right to me:
https://www.youtube.com/watch?v=SWZQsTUYuNw
If I can get enough flotation without the pontoons showing up in the video, I'll be happy. I know I am giving up some aerodynamics. I would not fly with the pontoons other than on a calm day because of the sail effect of the pontoons, but I also wouldn't fly over water anyway on a windy day.
I like the idea of the inflatable "float bags" that someone suggested, which would inflate after being submerged and keep your drone near the water's surface. But, while they might be more aerodynamic than foam pontoons, they would't leave any chance at all of a safe landing and takeoff on the water. Where I live on the Chesapeake Bay with brackish water, it is rare that electronics can be rinsed, dried, and returned to service after being submerged. So while that might work in fresh water, I don't want that risk where I will be flying.
Also, after watching several videos of successful takeoffs and landings with foam pontoons, it seems like there's at least a 50/50 chance or better of an upright, safe landing on the water even in a low battery situation. And if you do flip it over on landing, you are no worse off than you would be with the "float bags" which guarantee being submerged.
The foam noodle pontoons, secured with zip ties, would be easily removed when not needed. I'm adding a gimbal mount for the P2V's camera for image stability and camera control, so hopefully that will reduce any added unsteadiness from the pontoons catching the breeze.
I'm planning to do almost 100% of my videos over water, so a workable solution that adds a little bit of risk mitigation seems like it's worth a try. If the pontoons turn out to be too much of a wind drag, I'll just go back to flying without them.
Other thoughts welcome. Thanks to everyone for posting and contributing to this thread, very helpful!