Hi,
I fly mine into the sea too. I lost mine (p2 with fpv gear and go pro camera..ouch! ) into the sea, due to an error of using a weaken battery.
The battery weakens if it has gone through many charge and discharge cycles or overly discharged. A bloated
battery is a clear sign. The battery may not bloat but it does not hold voltage ...this is the more perilous one.
You can notice that when your iosd mini shows momentary drop in voltage and your P2 losses height.
That is when the battery discharges faster than normal. Imagine you are out there over the water while this happens.
Once you notice one of your battery has exhibited momentary voltage drop, keep that for calibration use and dont risk
flying with it. I had not found a good way to anticipate a bad battery other than looking for bloating signs.
Dont mean to pour cold water on your joy of flying over waters. Make sure you fly with a known good battery.
Mark the bad ones so you don't accidentally use it, like me. $$$at the bottom of the sea. Corrosion happens almost
immediately in sea water.