Which is why you could use an accelerometer. This would allow you to have a hermetically sealed electronics bay.
You can have a hermetically sealed electronics bay and record altitude as well with a diaphragm. Redundancy is good with electronics. Both would work, I'm just suggesting a way to use an altimeter as well.
How fast would ocean currents carry the rocket and how fast would the wind above the ocean carry the rocket?
(Once it lands in the water, since the chute is deployed, the currents will have more tug on the rocket.)
The water *might* seep in through the nozzle and slosh inside, if you have large freshwater lakes nearby it's a much lower risk since it's not corrosive (maybe a little?) and higher there's a higher chance for recovery as it's for the most part contained and there should be less wind since there's a smaller area.
You will almost certainly want a GPS tracker for large rockets, even with the balloon beacon. Otherwise, the balloon will eventually deflate, and the rocket will be pretty much gone.
Will the GPS tracker antennae need to be outside the rocket? If so, maybe some 316 (I think, it's for marine environments with corrosion) stainless steel rods along or around the body tube somewhere.
Will the salt water corrosion from a week of salt water sloshing have more or less damage than filling up the casing a quarter up with nails and shaking it like a can of paint?
Will the solid combustion products in the motor after launch dissolve into the water? Will they be in any way contaminate the aquatic ecosystems or water?
Fiberglass is a good choice. Everything on the outside surface after ejection has to be waterproof.
What about salt water on steel eyebolts or quick links? Galvanized steel? Stainless steel? (Problably going to be 304 stainless)
Remember that each way and each solution has it's own pros and cons.