I made this excursion 2 weeks ago.
You will meet your group outside the ship.
You will walk to the catamaran for about 10 minutes tops.
Basically, the catamaran is right outside the terminal across from the market.
There is only 1 bathroom in the catamaran and I doubt there is enough room in there to change.
All the folks had their bathing suit on prior getting on board. If you stay outside during the way back, you will probably be dry by the time you get back.
I google earth it, and the most you will walk will be 3,300 Feet or 0.63 miles.
The longest walk is from the cruise ship to the terminal. The catamaran is just 950 feet from the terminal.
Enjoy it.