I can't agree, Steve.

The question involves safety not electrolysis. It is well understood
that there must be no connection between the grounds to eliminate the
electrolysis currents and that is the primary reason for installing a
transformer.

However considering the safety question, the concern is the (remote)
possibility of a short from the primary winding to the frame. I
disagree with your statement that a fault on either side of the
transformer will trip the circuit breaker - that is incorrect.

If, as has been suggested, the transformer frame is connected to the
boat ground and you develop a fault from the primary winding to the
frame - and hence the boat ground - and hence the underwater metal, you
will NOT draw enough current to trip a 30 or 50 amp circuit breaker.
The underwater metal will be alive at a high AC voltage and
considerable current will be flowing to the water generating all sorts
of nasty gasses but there is no way the water is going to carry enough
current to trip the breaker. A swimmer in the vicinity is then at risk
of a lethal shock.

As Chuck pointed out, a GFI will disconnect as soon as it detects
leakage current to the water but GFI outlets on the dock are very rare
and GFI protection on the boat input side of an isolation transformer
is virtually non-existent.

A short from the secondary side to frame, if the frame is connected to
either shore or boat ground is a far less critical situation since the
secondary side is floating so no lethal voltage would be present on the
underwater metal in either case.

Regards,

Andina Marie Foster,