Just follow each wire to its connection point and test. You cannot electrocute yourself with three batteries even in parallel (the max voltage will only be 36V) If your that concerned slip on a pair of latex rubber gloves and get crackin.
I think you're reversing the terms "series" and "parallel," unless they're different in nautical electrics. Batteries wired in series, i.e., positive on one battery to the next battery's negative, then that battery's positive to the next negative again (and continue the chain), ADDS the voltage of each battery. That's the way the typical mutliple AA battery holders work; 6 1.5v AA batteries yield 9v when wired in SERIES. Three 12v batteries would yield 36v when wired in series.
"Parallel" wiring is connecting all the positives together and all the negatives together. This is done for increased current availability, but the voltage remains the same. When a typical marine battery switch with settings for battery 1, battery 2, or Both is switched to "Both," it wires the two batteries in parallel so that the voltage remains at 12v.
Hope that helps a bit.