If you are only going to be swimming at the surface of the water, you can use a swim buoy with dry bag, like this one:

https://www.amazon.com/New-Wave-Swim-Buoy-Triathletes/dp/B0170REZE2/

I wouldn't try to change batteries while floating in the water though.

Thank you - is this what you mean?

https://www.amazon.com/dp/B0170REZE2/ref=cm_sw_r_api_i_SBHZX6DY6R0A3X3C6XTP_0

It's pretty shallow and not any boats, but this does sound practical.