What do you need help with. Is it the electronics asspect of the design (senses etc.) or the construction asspect (shape, size etc.).
If you need help with both the electronic design and construction, I think you should approach each sepretly. First, build a underwater craft with a remote controll (can be teathered), then try to add electronics.
Also, if this is your first robot, I also suggest that you start by making a wheeled robot on land. This would be easier because it only needs to move in two dimensions (as opposed to three with a under water robot).