110v outlet on 220v circuit with 220 GFCI to boat dock
Don't think of it as a 120V receptacle on a 240V circuit.  Think of it as a 240V load on a multi-wire branch circuit.

That is a legitimate wiring method.  MWBCs can also supply 240V-only loads.  

There's one gotcha.  You can't have any receptacles on a circuit where the hardwired loads account for more than 50% of the total circuit ampacity.  That doesn't apply to cord-and-plug connected loads, so the workaround is to make the hardwired loads not hardwired anymore.  Fit sockets, cords and plugs.



So you have an MWBC.  If you employ a GFCI on an MWBC, you must do it with a 2-pole 120/240V GFCI breaker (or deadfront that is inline). You cannot use a European 2-wire RCBO, because all 3 wires, neutral included, must go through the GFCI's internal sensor together.

In any case, in a water related installation, you want the GFCI protection well before you get anywhere near the water. Because you want the wires protected also - if the wires get wet, they are protected, rather than electrifying the water and causing drownings.