The bus/coach ride from the Punta LaGorda pier to the ferry is about 30 min. and the coach is modern and air conditioned. The drive along Cozumel's downtown waterfront area and then through the jungle is very scenic and the guide is passing along interesting info so the time passes quickly. The small, open ferry then takes an additional 15 min. to cross over the mangrove to Isla Pasion. There is no bathroom on the bus or on the ferry but there ARE bathrooms on Isla Pasion so if you can last about an hour to an hour and a half between bathroom stops you can easily make it from your stateroom to the island's facilities.
And while we're on the subject: I always take an Immodium or two the first day of my cruise as a preventative measure. Basically it relaxes the colon so it is less reactive to the rich shipboard and Mexican foods and alcohol, and you will be less likely to have an urgent need to "go" at an awkward time like during a tour. It's also a good idea to take ashore a one-liter bottle of spring water each day you are in port. (Carnival automatically provides this in your stateroom for a nominal fee and will restock it every day if they see that the bottle is empty or missing.)
And don't forget, it's perfectly acceptable to pee in the ocean! Enjoy!!