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
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 currentcarrying
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.)
11.7.2.2.1.4. The shore-grounding (green)
conductor is connected, without interposing switches
or overcurrent protection devices (See E-11.5.3.5.),
11.7.2.2.1.4.1. 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.
11.7.2.2.1.5. If an optional
galvanic isolator is
used, the shell of a metallic shore
power inlet shall be
electrically insulated from the boat.
11.7.2.2.1.6. If the boat's AC
electrical system
includes branch circuit breakers, the branch circuit
breakers shall simultaneously open both current
carrying conductors unless a polarity indicating device
is provided. (See E-11.12.2.6.1 Exception.)
See Diagram 1 on page 11 at:
http://www.abycinc.org/committees/.%5CE-11.pdf