Capt bill1, yes you can do that, if you have the adaptor, but, if comes back reversed, which is it, the chicken or the egg?
Iron works, yep that will do it, because the hot, in a properly laid out panel, will come from the same phase, or more simply, the same side of the panel. 1 and 2 should be on the opposite sides, and 3 and 4 should be on opposite sides, using a 1 and 3 or 2 and 4 combo, there will be no differential between the lines. This is one thing GFCI's don't like, you have a different potential between the two lines feeding the panel, thus 240vac, if the potential was the same, it would not read any voltage between the two legs. You can prove this with a meter, test hot to hot on plugs 1 and 2, then 3 and 4, they will read 240, test between 1 and 3 and 2 and 4, they will read near zero.
32 carv, looks like everything is in order, and you have a nice looking set up by the way. There are a couple different ways to fix this. Using different outlets is one, if that is available to you. Using an isolation transformer is another, but expense and weight are big factors, but this would be the code worthy fix. Here is the fix if it were my boat, I would isolate the grounds and neutrals from each other. You have one circuit feeding the normal house loads. You have another circuit feeding nothing but the ac. Each of which have a ground and neutral from shore power. I am going into this with the assumption you do not have a gen set big enough to run the ac when not on shore power. Two methods of this, depending on panel type. Method one, remove the bonding jumper from the two neutral busses and the two ground busses, if you have two. This will separate the grounds and neutrals between the two while still giving you the same level of protection. Or, if you do not have separate busses, locate the grounds and neutrals from the ac shore power supply, and the ac units, wire nut the neutrals from incoming and out going together, and wire nut the grounds together the same way. This will isolate the grounds and neutrals from each other while leaving the primary circuit protection in place and not defeating the GFCI protection. There is nothing in the code that I can think of that specifically prevents this.
Downside, if you have a genset big enough to run the entire boat, this wont work, not with out a bit more work. If you do have one, let me know and I can give you some choices there, none of which will be cheap and easy. Also, with the wire nut option, you have another place for bad connections or corrosion to happen. If it works for you, you can look into other type connections, or best solution is to install a sub panel for your ac loads, bit more expense but a better solution in the long run. It is unlikely you will be able to convince the marina to remove the GFCI breakers..........D..........
