Rocket for schools in Sheboyagan Wisconsin has been launching over the Lake for years. K-550's and J-350's in 4in & 5.5in, rockets.
The rockets are launched off a pier and angled out into the lake. Coast Guard auxiliary retrieves them. They use Loc phenolic tube 5.5in wood fins ands plastic NC's. Usually 50-60 rockets are flown with no ill effects after floating for 10-15 minutes. Usually several
https://www.rockets4schools.org/ Fiberglass flights with L-M motors participate. Don't do anything out of the norm, but a water tight or foamed NC.
I have a 3in. rocket [fiberglass]I fly at Bong, with a foamed fiberglass NC, carved out for tracker, nothing else modified. twice it's landed in the lake for over an hour, I find it ...NC bobbing around on the surface, like someone fishing .......sending it's signal...rest of rocket sunk below surface. Swim out to it...retrieve, rinse off altimer with bottled water, set on dash to dry out, ready to fly next day.
I have foamed a straw alongside the edge of NC for antenna and press fit the tracker[carve space with knife] after fully wrapping with electrical tape to waterproof it. You can water-proof the av-bay by flush mounting aquarium pump hose in the vent holes, then making a small coil on the inside before the open end. It will breath fine before splash down.
Works kinda like a P-trap, some water may enter, but can't make it's way around the loops to the open end.
I've landed in water over a dozen times.... not by choice. Rinsing and drying some altimeters many times, so I don't even bother with water proofing ac-bay anymore. These were MissleWorks and Perfectflite altimeters.
