She's right, they have bins next to the beach. It's the last thing you'll get after entering the park. You reach into the bin, grab one, and go. The XXX sizes will still be there, that's not a problem. The Medium's go fast.
That's another reason why you want to get there early. They have a limit of 1000 people, but in the morning when everyone shows up at the same time wanting the same thing, you'll end up waiting if you get there at say 09:15am. Be there by at least 08:30am, I'd even say 08:00am. You may end up being bored for 45 minutes, but then you're first to be checked in, first to the snorkle station, first to the wet suit bins, and first to pick a spot on the beach.