This is true but only for house (i.e., land-based) wiring. Electricians used to working on houses who are not marine electricians often make this mistake.
As Jim said in the second post:
"The ac ground should not be connected to anything except the green screw on the ac outlet or ac switch and the metal of the circuit breaker box. If there is a short, the current will go back to the dock and hopefully pop the circuit breaker. There should be a protection device on each set of outlets. If an appliance shorts to ground you do not want 110v current going into the water.
The 12v is fine connected to the engine block since the battery also grounds there."
Don,
What you and Jim have said is incorrect as it applies to marine AC electrical standards. ABYC E-11 requires the AC green/grounding wire to be tied into the ships DC ground on-board the vessel and white/neutral and green/grounding should never be connected on-board accept at a "source". This has been the marine standard for many.
Info below from ABYC E-11:
"11.5.3. FOR AC SYSTEMS
11.5.3.1. The system shall be polarized as
defined in E- 11.4
11.5.3.2. A grounded neutral system is required.
The neutral for AC power sources shall be grounded
only at the following points:
11.5.3.2.1.
The shore power neutral is
grounded through the shore power cable and shall not
be grounded on board the boat.
11.5.3.3.
The main AC system grounding bus
shall be connected to
11.5.3.3.1.
the engine negative terminal or the
DC main negative bus on grounded DC systems, or
11.5.3.3.2. the boat’s DC grounding bus in
installations using ungrounded DC electrical systems.
11.5.3.4. In AC circuit, all current carrying
conductors and the grounding conductor shall be run
together in the same cable, bundle or raceway.
11.5.3.5.
There shall be no switch or overcurrent
protection device in the AC grounding (green)
conductor.
11.7.2.2. APPLICATION OF TYPES OF
SHORE POWER CIRCUITS
11.7.2.2.1. Single Phase 120-Volt Systems
with Shore-Grounded (White) Neutral Conductor and
Grounding (Green) Conductor.
11.7.2.2.1.1.
The shore grounded (white) and
ungrounded shore current carrying conductors are
connected from the shore power inlet to the boat's AC
electrical system through an overcurrent protection
device that simultaneously opens both current carrying
conductors. Fuses shall not be used instead of
simultaneous trip devices. (See E-11.12.2.9.2.)
11.7.2.2.1.2.
Neither the shore grounded
(white) neutral conductor nor the ungrounded current
carrying conductors shall be grounded on the boat.
(See E-11.5.3.2.1.)
11.7.2.2.1.3. When more than one shore power
inlet is used, the shore power neutrals shall not be
connected together on the boat. (See E-11.5.3.6.)
11.7.2.2.1.4.
The shore-grounding (green)
conductor is connected, without interposing switches
or overcurrent protection devices (See E-11.5.3.5.),
from the shore power inlet to11.7.2.2.1.4.1. an optional galvanic isolator, and then to
11.7.2.2.1.4.2.
all non-current carrying parts of
the boat’s AC electrical system, including
11.7.2.2.1.4.3.
the engine negative terminal or its
bus.