Hi, Asher ...
Pull on shorts and T-shirt over the B-suit. If you take undies, put them in a zip-top bag to keep them away from sand and suntan lotion.
There are ladies' rooms at the major beaches on Cozumel. In Playa, there's a decent ladies room behind the restaurant at the Costa del Mar. It's one property closer to the ferry pier than the Blue Parrot.
If you are Sting Ray-ing with Captain Marvin, your suit should be dry by the time you return to shore. Otherwise, there is a ladies' room. Gear is stored on board. StingRay tip ... take some sodas and snack food. All Marvin stocks is water and icky red punch. I don't know about the ships' Shorex. But, after a while on the water, the bait chum starts to look pretty good.