No matter how many ways you measure for stray electrical current in the marina, if you go in the water you are betting your life on chance events.. You are betting that the boat 2 slips down doesn't have backwards wired equipment that will cycle on while you are touching bottom, or touching your grounded boat... That no one will flip on the AC breaker on a motor wired backwards while you are swimming, etc... There are a thousand ways to die and jumping into marina water is one of the more sure of them... denny-o old industrial electrician