Preventing shock risk on isolated transformer secondary circuit
Note: The following is not as complex as I've ended up making it look. Everyting I've said (hopefully) applies - but look at the diagram, trace out possible current paths and it should be clear enough.
If an RCD's go &amp; return currents differ it will trip.

As "phase 2" is essentially "local ground" - including the water, general metalwork and potentially any wet surface - then great care is needed.

I will call the grounded phase2 to vessel metalwork "hull".

In some (many?) marine systems both phase1 and phase2 are floated relative to hull and the existence of any hull-to-phase connections is monitored. Any such connection is sought out and removed rapidly as it otherwise allows the situations mentioned below. 

An RCD will work if it is used to monitor go-and-return current (phase 1 to phase 2 current in your terminology) and you touch phase1 accidentally and also any "grounded" item that is NOT in the RCD return path. 

However, if there is a phase2 to hull fault a shoch may occur from phase1-user-hull-phase1 fault - rcd phase2. There will be no protection against shock.
An example in the diagram below is eg a shock from CD to hull, and a phase 2 fault to hull at GD. RCD2 will see the same go and return currents with shocking results. 

If all powered devices on a given 'stub' beyongdan RCD scrupulously isolate / insulate their phase 2 returns from vessel ground then an RCD will protect you against unintended phase2 grounds on other circuits. Any phase2 to hull shock currents will bypass the RCD as their is no stub-phase2 return path. 

The RCD protection from a phase 1 contact fails when any other device on that RCD circuit has a phase 2 to ground fault (connection). Current will then flow from phase1, through you to ground, from ground via the second fault into phase2 then back through the RCD.

If a single RCD or few RCDs protect the installation then you a device with a phase 2 to ground fault becomes a significant hazard. If there are many RCDs - so a given RCD protects a short circuit stub, then the RCD protects against any phase2-ground fault that is NOT on its stub circuit.



In the diagram below Phase 2 is hard connected to hull.
Phase2-I (to right of transformer ) is a system with no formal hull connection.
GA - GE are points of possible connection from phase2 at that point to hull, regardless of whether the system has a formal phas2-hull connection or not.
CA-CE are possible contact points from a user to phase1.
RCDs 1-3 protect wire pairs passing through them. 

A CA-hull shock is unprotected against a GA fault, but
A CA-hull shock IS protected against a GB fault.
A GB fault will trip RCD1 when a load is connected even when there is no CA contact.

CC, CD, CE contacts with no GC, GD or GE faults will not give a shock to hull. 

CC to hull contact with GC fault  is NOT PROTECTED - this is obvious enough to not be a surprise.

Other paths are generally protected (unless I missed something). Trace the path of current starting at the phase1 source - "out" of the circuit at Cx, to hull and then via Gx to phase2. If the go and return path do not pass through the same RCD the user is protected. 

As a "bonus"- and a source of confusion:  


CD-GD &amp; CD-GC both trip RDC2  
CD-GE trips RCD2 &amp; RCD3 simultaneously.
GD 
&amp; GC probably trip RCD2 if there is any load present, even though there is no "shock" occurring. This is because some of the phas2 return current flows via the RCD and some via the double fault 


Similarly GE &amp; GC trip RCD3 when load is applied via RCD3. 
Similarly when GA &amp; GB occur. 



Without RCDs 


CA (or CB) shocks to ground will occur without any phase2 ground fault.
Any CC CD or CE shock to hull is not protected if any of GC GD GE faults exist.








simulate this circuit &ndash; Schematic created using CircuitLab