Submarine project
Have you considered using simple acoutic signalling through the water? Easy to make hydrophones would allow long-distance comm link between your sub and the surface. Modulated control signals on a audio signal are easy to do, and give you full control of your model in he water. Sound pick up can be microphone literally glude to inside of plastic shell. Be aware that your hull shell will be only a shallow depth unit, as the water pressure will enter any space, seam, or connection it can. You have an interesting idea, and what looks like a great way to explore. Consider a tethered robot sub first to help you work out all the bugs first, and to make recovery easier if there is a failure. By the way, consider a sonar link for transmission of video, and put a camera in the robot sub.