Some time has passed since the original post, but I'd like to add something that seems like it could be significant, albeit rather strange.
I'm a vacation diver with 88 dives spanning 9 years, but today I experienced for the first time something similar to what the OP describes : swollen uvula, to the point were swallowing is a little bit hard. The feeling is highly unpleasant. The only couple other times in my life where I've had this condition/sensation was after sanding drywall compound without a mask. If remember well, the sensation disappeared after a day, maybe two.
There are two circumstances worth mentioning, the second is the intriguing one :
1 - I've done 8 dives on my current trip. As always, I used rented equipment, but this was the first time I was provided with a regulator that let zero water in. I guess it was in good condition and perfectly adapted to my mouth. I always thought that having a little water seep in was mildly annoying, but now I realize that it has the beneficial effect of keeping your mouth wet. On pretty must each dive this week at some point I would purposefully let a little water in to help with the dryness. Not sure If I did this morning, perhaps not and this could an explanation, at least partial, for the swollen uvula.
2 - Quite strangely, this morning I was diving at the exact spot at the OP, i.e. Palancar Caves in Cozumel! This thread was one of the first Google hits I got after typing something like "scuba swollen throat" so I was kind of shocked with the matching diving spots. Of course it could be a pure coincidence, but I thought it was worth posting.
Ocean waters are made of myriads of things and can have peculiarities. I've had another weird oceanic occurrence in this part of the world when after a dive the silver bracelet that I wear all the time turned completely black. That was in Playa del Carmen, just in front of Cozumel. My bracelet returned to normal after some time, possibly due to friction, and never turned black again. In that specific case, it's hard not to think that something rather unique in the water reacted with the silver...