I just went through that on a DVC night at AK last week. Me and a friend were asked "can we seat you with someone else?" and we said sure. Sure enough we get a wife and her very large husband. I was leaning out of the boat for the entirety of the ride. I was actually smashed into the side of the boat.
And like I said, Moonlight Magic night aka no rush and 5 minute waits. Completely unneeded.
Point of reference, I'm 160lbs and 5 8"-9".
And if they do that, I'm going straight to GS. It was beyond uncomfortable and well into pain territory. I'm fine with being pressed up against someone but once I can't move my arms and legs and am being forced against the edge of a ride vehicle, I'm done.
Also worse because this attraction will have one solid lapbar, which always means there is a chance of someone thinking it is more than a deterrant restraint and needs the security of it touching the knees of the smallest person in the row.