I'll try to help out, let me know if you have other questions. We just did this in January for our 15th anniversary. I surprised DW with it and it was awesome!
They do only 2 per cruise, but we were able to book it about 5 months early with no problem, I believe we were the only ones on our cruise.
Ours was on a 4 day cruise, and they did it on the Bahama day. We didn't have a choice, but this was perfect for us anyway.
It was done in the lounge on the main deck, where you enter the ship, can't recall the name of it, but it was very nice...private, roped off no one but us and our kids, the captain, the concierge and the piano player, and the photographer.
We got to pick out flowers and the cake topper.
We did add on the photographer package. She took a bunch during the ceremony, then took us around the ship for another hour taking great shots, expensive, but worth it to us.
I wore a suit, DW wore a nice dress, but not a wedding gown and tux, they will rent tuxes, but we didn't feel that was necessary.
If you have any other questions let me know. I thought it was awesome and well worth the price, I know my DW would agree!!
Mike