I do a lot of flying over water. I looked around at different flotation ideas. They all seem like they would affect aerodynamics. So I came up with this. I glue a Velcro strap to the Water Buoy (product name) for stability but also use a small wire for security. The WB will float 1kg. Mavic Air 420g. It inflates and has a light come on when immersed. I experimented with mounting on the top and bottom of the quad. Both work, but mounting on the bottom makes hand launching and landing more difficult and on a boat that is by far the best option. This will not keep you MA dry or even usable but you can get it back and use your DJI Refresh. I HAVE NOT had to test this thankfully.
Happy Flying
Happy Flying