Where domestic water heaters and other household appliances, and marine AC wiring in general, are concerned, neutral to ground connections should only be made aboard a vessel in select locations, at a genset and inverter, when and only when they are the source of power. If wired properly, this is automatic, no user input required. If the vessel is equipped with an isolation/polarization transformer then a neutral to ground connection is made there as well, with one difference, it's continuous because the transformer behaves like a power source whenever the vessel is plugged into shore power. Otherwise, under no circumstances should there be a connection between neutral and ground anywhere else aboard. This often confuses non-marine electricians in that they are accustomed to bonding neutral and ground busses in main panels. If an inappropriate neutral to ground connection is made aboard, then the water in which the vessel floats becomes part of the return path, which in turn creates an electrocution potential. This bond can also bypass a galvanic isolator, which increases the likelihood of galvanic corrosion, however, the corrosion is not a direct result of the shore power, it's simply a side effect of the neutral to ground connection fault.
This applies, once again, to domestic, non-marine water heaters and other appliances.
