I have some wild notions that might advance RC submarines.
 

Though I don't yet know much about them.

Radio reception is weak to none under water when diving deep. Another technical way that could communicate with underwater subs is pulsating encoded sound signal commands which could control them. Sound carries with much superiority underwater than does radio waves. Pulsating light signals, IR, is a common way that TV remotes work.

Sound is the way underwater sonar technology works and how whales talk underwater to each other.

Why not send a sound signal underwater to control boat model equipment? Radio waves might still be used when the sub is surfaced.

The sub could be a hybrid RC/SC sub. Combination radio/sound-controlled.

A camera can be mounted on a sub to see underwater but it is hard to keep one's sense of direction or bearing underwater. Subs might include on-boat instrumentation as an electronic compass that could theoretically send back information as bearing to the pilot via sound waves.

Could model subs even be equipped with GPS?

Radio waves don't travel well in water. For US subs to communicate, they have to use a very low frequency band. This would make GPS basically worthless underwater, as is 2.4 for the most part