Hello!
I have a Mavic myself. It does not give the impression that it is waterproof, and the manufacturer does not claim this anywhere, as far as I know. I wouldn't expect it to work after a submarine mission, the optics and the active cooler, as well as the sensitive gimbal, will certainly be damaged.
Greetings Dominik

Basically, you can assume that drones are not waterproof, unless this is explicitly stated by the manufacturer. Most drones are not even splashproof, but some can be made so with an additional protective jacket.
Since as much weight as possible has to be removed from the flying body, no seals are attached to the battery, gimbal, etc. The most dangerous drones are the rotors. The rotors are even more dangerous. In most cases these are open at the top and only a little water destroys the electric motor.