Marine AC - Why Tie Neutral and Earth?

[Wotname]

Can someone explain the logic of using a tied neutral/earth AC system on a sailing boat?

Is it simply a NEC code requirement or is there some inherent safety benefit in not having a fully floating Active and Neutral AC output?

I understand why shore power uses a Neutral / Earth connection but why do this with an onboard AC supply?

Regarding using shore power on board, surely a safer system is to isolate the shore side from the boat side with an isolating transformer thus creating a fully floating A&N output onboard.