[efnx]

What about a fully autonomous submersible that surfaces periodically to get its bearings from GPS, upload data, recharge through solar, etc... It could be programmed with a certain number of goals and priorities and follow the goal with the highest priority given some parameters.

[efnx]

Or, what about using low power radio to communicate relatively short distances to other submersibles scattered upward toward the surface, forming a mesh network that eventually has an endpoint connected to the ship or satellite?

[panacea]

Poseidon Exploratory Advanced Research Laboratory. PEARL.

A string of submersible robotic submarines linked by really long cables that are dangled into the ocean.