From past experience we prefer to be by ourselves or one other couple and would prefer multiple boats. This allows for greater autonomy of what people get to do. 3 groups of people wanting to do different things is more hassle than 2 groups etc…
I think you should stick to your guns even if couple #2 wants couple #3 to come along. True, they may be otherwise lovely people and you may end up being the best of friends someday, but it won’t be on your terms.
If you allow them on this trip you have set a precedent that says they are allowed to horn in on any all future plans you may ever make. It will become harder and harder to say no another time if you don’t do so now.
Draw one of them aside and explain that you and the other couple have agreed that your marital vows only apply on land, so to join you they’d have to agree to your “anything goes” condition. If they still seem interested, up the ante: as far as the guys are concerned, anyone over 18 is fair game, of either gender. If that doesn’t scare them off, well, that’s just incredibly bad luck. Unfortunately now you both have to invite them and have group sex with them.
But that’s pretty unlikely and, IMO, well worth the risk.
Hey, that’s not so bad afterall. The wives of couple #2 and #3 are both pretty hot! But seriously, I’m going to do the “rent your own houseboat if you want to go” thing. It’s a no risk proposition
I did a houseboat trip once with a whole bunch of work friends more than a decade ago, and while it worked out great, there were numerous issues with food (a vegetarian, person who doesn’t eat pork, and person who doesn’t eat fish). Then there was the problem of people making noise while other people tried to sleep, and there was some random nakedness, which is NEVER a good idea with work people, and doubly so when one of the women was a bit prudish. I remember it still being a great time, but having to be the ‘den mother’ as the organizer was a headache and I did not like having to settle issues between friends.
George Costanza would call this “discovering plutonium - by accident.”