There is a TFFer that goes by Jiggin El on this forum that is a Tubing Guru. He showed me the way to go in Tubing & I`m sure he can help you like he helped me. What kind of tube are you going to be fishing out of?I have shot a dock or two before from a higher platform but a Float Tube
Unless you can get completely under the dock, Why wouldn't you just get close enough to the dock & just stick the rod under it?You are already at water level