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My understanding..
The idea behind the green wire in an AC system is that anything which you could get shocked on (such as a metal chassis or the DC ground system in the boat) is electrically connected to "green" earth ground. If somehow, the AC hot wire gets accidentally connected to something which you could get shocked on, and the green earth ground is implemented, a lot of current will immediately flow and the fuse supplying the current will blow and remove the hot AC. Without the green wire protection, if the hot AC wire accidently got connected to for example your boat DC ground, no fuse would blow and you would have a safety risk and possibly significant because your near water. You may not even know everything on the boat is "hot" since things still seems to work fine (except maybe if your prop is connected to the ground then you might notice the electrolysis boiling action going on under the boat until the prop soon turned to dust)
GFCI is different but also needs to have the green ground wire to work. Normally an AC load has all the current flowing in the hot and neutral (black and white for 110) wire. The GFCI is checking to make sure the currents are equal in the hot and neutral. If for some reason even a small amount of the hot current is diverted to Earth (green) ground, the GFCI will very quickly trip. An example of how this works would be if your barefoot standing on very wet lawn and are using an AC weedwacker - possibly a beer in the other hand. If the AC cable was frayed and you touched the hot wire, some of the hot current would be diverted to the wet ground your standing in though your body. Since the hot and nuetral currents are now not equal, the GFCI outlet would trip removing the hot AC. Seems to me lawyers are partly responsible for GFCI being needed in a boat as the only time I can see GFCI doing anything is if you have AC shore power plugged in, your boat is sinking so filled with water and you standing in the water touching the AC hot wire at the same time. Why is the boat sinking - maybe you just took a direct lightning strike just before you decided to grab the hot AC wire. As correctly stated already, GFCI outlets have a input (line) and a load outlet. Any AC outlet or device connected to the load line is protected by the single GFCI outlet.
At the marina, the green earth ground is likely connected to the neutral line back at some breaker box and also, the green wire is actually at earth potential because its connected to some sort of underground metal structure. So the green wire at the breaker box is likely also at the same potential as the water.
If everyone wired boats using the marina AC perfectly, all the AC current would be carried on only the hot and neutral wires - there would be no current on the earth ground wire and it would always be at the same potential as earth and water. But, you can easily use the green earth ground wire to carry current similar to the neutral wire since it is likely connected to the neutral wire back at the marina switch. So its an easy mistake to make as everything seems to work fine and you still actually get the safety protection of the earth ground wire.
But.. since the green earth wire is now incorrectly carrying AC current back to the marina fuse box, there will be a voltage drop on the green wire "net" because the wire has some resistance (ie, current and resistance = voltage drop). If you have a slip that lives somewhere on this net, because the current flowing in the green wire and the induced voltages, your outlet green wire will NOT be at the same potential as the water. So if you connect the earth green wire to structures on the boat which are in contact with the water, you have created an electrolysis problem because the green earth and the water are not at the same potential.
The simple solution is to put in the diode isolator which allow up to about 1.4 volts difference between the boat DC ground and the AC earth ground and you "hope" that your slip AC Earth ground has less than 1.4 volts difference to actual earth potential. But you still get the same safety protection because if you accidently short the AC hot wire to the boat ground, the diode isolator becomes forward biased (i.e., turn on), lots of current flows and a breaker trips.
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