To feel tingling with DC you must somehow be completing a circuit between pos and neg with your body. You can touch either pole individually and not get a shock. So at the time you were feeling the tingle from the chain, what else on the boat was your skin touching?

If you had a direct short between pos and neg wires you would not feel anything yourself, the current would pass thru the short (and hopefully trip a breaker before it starts a fire!).