Might be good to note that the posted diagram is a schematic of the electrical, not an actual diagram showing how the boat is actually wired.
As
@Kingjim91 stated for his boat, there may not be a junction block, the wiring probably goes directly to the AC breaker.
Also note that the outlets are daisy chained, meaning the AC wiring runs from the breaker into an outlet, then out to the next outlet, then into the last outlet. Probably to the galley outlet, then to the amidships port outlet, then to the head.
So, the amidships port outlet probably has two wires coming out of that outlets electrical box, one from the galley and one to the head outlet.
Be sure to unplug the shore power before working on the wiring and never trust what changes a previous owner may have done.