Fair point. But GPS signal from a submarine is almost impossible considering GPS needs LOS.
LTE has a range about 10miles and 5G is also LOS. So its brings it down to unlikely that an Apple watch could connect to cellular.
Considering this is underwater and radio waves attenuate very quickly in the water, this is very unlikely to produce any valuable tracking as a majority of the packets would get dropped if any make it to the cellular tower at all.
Only real way someone could track this submarine via cellular would be if they used a cell site simulator and downgraded the signal to 2g, which would be possible with Chinese cellular providers.. But this would require already knowing where the submarine is and/or having stingrays all over the ocean.
Unless there's a super hacker or NSA agent in the class that can figure out your password in real time.. You should be fine doing that.