Assuming your dock has a proper 30A 120V Twist-Lok Shore Power
Outlet (receptacle), one simple solution would be to:
Your 30A (3 - #10)cord will plug
into your 30A twist-lok Inlet (on boat)
Wired (3 - #10AWG) to a 2Pole 15A Breaker, wired (3 - #12 or #14) to a 15A GFCI Recptacle, then to the second standard receptacle.
The Black Hot wires are connected to the breaker and brass terminals.
The White Neutrals are connected to the breaker and silver terminals.
The Green Ground wires are connected to the green grond terminals, which are isolayted (not connected to anything else).
Excerpted from ABYC E-11
184.108.40.206.1.1. The shore grounded (white) and
ungrounded shore current
carrying conductors are
connected from the shore power
inlet to the boat's AC
through an overcurrent protection
device that simultaneously opens both currentcarrying
shall not be used instead of
simultaneous trip devices. (See E-220.127.116.11.2.)
18.104.22.168.1.2. Neither the shore grounded
(white) neutral conductor nor the ungrounded current
carrying conductors shall be grounded on the boat.
22.214.171.124.1.4. The shore-grounding (green)
conductor is connected, without interposing switches
or overcurrent protection devices (See E-126.96.36.199.),
188.8.131.52.1.4.1. an optional galvanic isolator
184.108.40.206.1.4.2. all non-current carrying parts
the boat’s AC electrical system
220.127.116.11.1.4.3. the engine
negative terminal or its
18.104.22.168.1.5. If an optional galvanic isolator
used, the shell of a metallic shore power inlet shall be
electrically insulated from the boat.
22.214.171.124.1.6. If the boat's AC electrical
includes branch circuit breakers, the branch circuit
breakers shall simultaneously open both current
carrying conductors unless a polarity indicating device
is provided. (See E-126.96.36.199.1 Exception.)
See Diagram 1 on page 11 at: