once you get into a marina you can't imagine all the water hoses and electric wires that are running around to service the boats. each of those slips runs wires to a central location where all the meters are located. Each of the wings are tied together with structure about 4/6 ft under the water. Next there are various huge concrete weights at various points up and down the dock that are connected to the docks by wire cable. No there are also gasoline and diesel lines running under the docks. With all that going on aound a marina then imagine your a diver having to service all that stuff and contend with broken hooks and fishing line in next to zero visibility.
Please stay away from the marinas there is a lot more going on under that dock then you would ever imagine.