Welcome back Molly!
The
Xel-Ha Park and River Tubing Adventure looks to be an amazing one. As you see on the Port Adventure description, you'll have a bit of travelling to do to get to Xel-Ha, including a ferry boat ride and then a 45-minute motorcoach ride both ways. The motorcoaches that are used do not fit car seats as there is not a place to strap them in. So, this is good and bad, right? One, you won't have to worry about hassling bringing a car seat to then lug around with you, but secondly, you want to be sure your little guy is safe. I completely understand. However, every single time, we've found it to be very safe and comfortable for our kids. Though I don't know what exact type of motorcoach you'll have as it's always subject to change, you can expect that it will be a nice ride. Simply plan to have your son sit on your lap and you'll want to have a day bag with you that includes some snacks and activities to keep him engaged during this trip. Definitely bring an easy umbrella type stroller so it's quick and easy to fold and store.
However, if all that traveling might be too much for your little guy, and since he is too little to do too much at that park, you might instead consider the
Cozumel Beach Break. There is something for everyone here with a much shorter ride. Only 25 minutes away from the ship you can relax on the beach, eat delicious food, and play in the waves. Your little man will love the kids' waterpark area there and the older family members can thrill in a ride on one of the larger slides too. My family loves the Playa Mia Beach Club, so this I can highly recommend!
Happy planning!