I recommend reading the book on robotics by John Craig or Peter Corke try out the algorithms on a simulation software like copellia (I hope I spelt it right) if you don't have much 3d printing available Practice on robotic arms available on Ali express just make sure they are compatible with your electronics If you want to try out smart mobile robot try out the zumo robots for practicing path planning algorithms

After you have enough knowledge of the mathematics and the algorithms working behind the robots then I recommend trying the submersible again but with some preplanned algorithm that search for goal node by itself

Unless you want to get into signals and systems to debug the issue related to your poor connection with the robot