I think it's so cool you could in theory buy a satellite module eg. $250 break out board from spark fun, plug it in and be talking to a robot in the middle of the ocean. I wondered about the legal aspect, can you do that? Just release something into the wild.
CUREE, the Curious Robot for Ecosystem Exploration, is another autonomous underwater vehicle built around the Raspberry Pi 4: https://warp.whoi.edu/curee/