Charging battery voltage question

[Siddo]

i was charging the battery that is in the boat this arvo which is a 12ahr SLA battery and the charger is one of those jaycar multistage chargers. never noticed this in the past but went past the boat and brushed my shoulder against the side of the boat and felt a bit of a tingle. boat is a quntrex tinney painted white. chucked the multimeter between the hull and a ground outside the boat and can measure 240-270 ish milliVolts. is this normal or am i corroding the boat away?
wiring is all isolated to and from everything via flex conduit and then into a waterproof box with a isolator key that controls the battery flow to the switch panel. the key is out as i remove it when putting the boat back on the trailer.

any ideas

cheers
ryan