Hobby sub you can trail a float and have an antenna on it. Flexible plastic coated steel cable to a fishing float, a constant tension spring on the submarine model and then a short whip antenna will get you the depth you want and still give somewhat usable communications, using standard radio control electronics. Just accept you have a horrid path loss, but will work, and if you use old VHF TV channel 2 as transmission band you can do standard NTSC video as well, and have short antenna length with off the shelf receiver and reasonable size antenna.
10m of steel plastic coated cable acts like a coax cable ( lossy though it is, it will still work though you might have 10dB attenuation on it) in water, and will at least work providing you use a long range transmitter ( higher power) on the sending side, and a long range receiver (lower noise, better data recovery and much larger AGC range) on board, and the bands are far enough apart that a reasonable compromise on separation of the 2 can be made with low cost components in a shielded RF combiner on the boat, and shore side you just need a tripod for the TV antenna and use a broad beam mesh VHF antenna to get the coverage.