Michael,
You're lucky to have so many options :-)
Of those you mentioned, I think the best antenna would likely be an insulated triatic stay, fed through the mizzen mast, with the tuner located as close as possible to the base of the mizzen mast, and with the antenna led through the deck.
If the steel beams under deck are affixed to the steel hull...as I assume they are...you can just tie the tuner ground to the nearest steel beam. That should make a dynamite antenna system.
You don't have to guess. Buy the roll of insulated wire I suggested earlier. AWG #16 or #14 would be fine. Run it up the mast -- outside is fine for the test -- and under the existing triatic to the truck of the mainmast. Just use nylon
line for
insulators (at the truck of the main mast and the mizzen mast). Actually, since this is a temp setup, you can just use dacron halyards to hoist the insulated wire.
Connect the lower end of the wire to the tuner antenna lug, the tuner ground to the nearest steel beam, and give it a try. Should work just fine.
Now, assuming all is fine, go ahead with a more permanent arrangement, using
insulators in the triatic and insulated, tinned wire inside the mast. Remember that the wire going up the mast is
a radiating part of the antenna, so the total antenna length will be substantial (30' or so for the triatic plus the mizzen mast height).
What you'll have, in essence, is an end-fed "inverted-L antenna". These can work very well. I actually have one at my home station, using an SGC marine tuner, which is a great performer.
Let us know how it works for you.
Bill