Mermaid dress here too. Since we will be near our hotel rooms at our resorts, my plan will be to take my mother/sister/then husband with me back to the room and take the dress off, pee, and then put it back on. Luckily my dress has a zipper (and a few hooks for the built in corset) so it will be faster.
I will hydrate well before heading to teh salon for hair and makeup and then pretty much stop drinking fluids until the reception. At that point if I have to go take the dress off once it won't be the end of the world, but if you drink all day then you will have to go through the effort multiple times. (And don't forget if you are drinking a lot of alcohol that once you break the seal....)