yes I was replying to the pre-edit post - referring to GI which made no sense.
but to answer the boat out of water using Iso transformer correctly. - arguably thats very safe as there is no way anyone standing on terra firma can get a shock off the boat even if the live is directly connected to the hull. when i was a spotty youth science labs in schools were all fitted with isolating transformers so kids couldn't get a shock - thats the whole point of them, we happen to use them on boats to prevent galvanic currents rotting our hull/fittings but thats not what they were designed to do.
but if your out of the water you shouldn't be using the transformer (no point) and then I'd agree I don't think I'd want to rely on the earth/CPC of whatever dodgy extension leads and mains distribution lash up the boat yard has...
ETA- realised Arnot has already made the same point more elegantly earlier on.