Love the Christmas card - great picture!
We have made ressies for Xcaret too, DS13 can't wait for this. Is lunch at the buffet there included, are lockers available, did your DD6 enjoy the underground river? We also have a DS6, this excursion is the reason for the Western crusie, I hope DS6 enjoy Pirate night and can stay awake. We also have main seating dinner.
I would love to see pictures of Xcaret.
Thanks for your tips, great advice.
Lunch is included at Xcaret. It is not a buffet though. You get a choice of a hamburger, hot dog or salad (I think) and a soda/water. You are given a coupon for your lunch.
Disney contracts a tour company that orchestrates this excursion. After getting off of the Magic we got on a ferry which was a very nice ferry and was right at the dock where the ship is. It was very large and air-conditioned with airplane-like seats. The water is a bit choppy and they passed around seasickness bags as well as free seasick medication (Bonine). Since 2 of us were already on Bonine we did not take any. We were all fine and I did not see anyone getting sick. The ferry ride is about 40 minutes. Once you get off of the ferry they line you up and walk you through the town (Playa del Carmen) and take you to a bus. The bus is also very comfortable with air-conditioning and comfy seats. The bus ride was about 15 minutes or so.
At Xcaret the tour company will gather you together and explain where to go. They encourage you to start in the underground river first. I think only 1 family decided not to do this. You are taken to an area to get a life-jacket (mandatory) and are given a large bag with a lock. You place all of your belongings in this bag and lock it and take the key with you. The key is on a stretchy band to wear on your wrist. Since there was 5 of us we needed 2 bags.

Be careful with the key, DH lost his in the river, he thinks he caught it on something and it cost $10 to have them open our bag. You give your bag to the attendant and they transport it to the end of the river. Once you get out of the river you get your bag. There are lockers very near the end of the river if you wish to then keep some stuff in there.
As for the river, the water is a bit cold. They say it is 62 - 67 degrees. It was not too bad, you just have to jump in. It takes about 45 minutes to float through the river and that is with swimming a good bit. If you just float it will take longer. It really is beautiful and a cool experience but our 6 year old DD did not like that she could not get out. She got cold and tired about half-way through. I should clarify - you CAN get out at various points along the river and warm up or take a rest but they told us that you have to get back in and complete the river. I am not sure if that was totally accurate but we just stayed in the river until the end and had a whiney 6 year old.
Once we got out of the river we got our stuff from our bag and walked 2-3 minutes to the restaurant where we were told we could turn in our coupon for lunch. The restaurant is a beautiful open-air restaurant on the beach. You can order other food and pay for it if you choose. DH and I shared some fajitas and we each got a Corona.

we also got our hamburgers. My kids got the hotdogs but did not like them. I tasted one and understood why. The hotdog was very different tasting, more like a bratwurst. Luckily I had brought some snacks from the ship. It started to rain while we ate so we ended up spending more time at the restaurant then I would have liked. We probably lost about 1/2 hour by doing this. After it stopped raining we went to the beach just outside of the restaurant and swam and snorkled. It is a really beautiful beach.
The only thing that bothered us is that there just was not enough time to do much at Xcaret. There are Myan ruins, animals (jaguars, monkeys, etc) as well shops, shows, and lots of other offerings. We only had about an hour and 15 minutes after lunch. Now, we would have had 30 minutes more if it had not rained. I do not know though if we would have done anything different. The park is huge. It is like a natural theme park. It took us about 15 minutes to walk (very quickly) back to the entrance from the beach where we were to be back for our meeting time for the bus to take us back to the ferry.
All in all it was a wonderful day. We all enjoyed the experience although I think the kids liked the beach even more than the underground river. The tour company that coordinated this excursion did an excellent job. We did not have to wait for the ferry or the bus at all. When it was time to go the ferry/bus were ready. I actually enjoyed walking through Playa del Carmen from the ferry to the bus (and back) and got some cool photos.
My husband is getting all of our photos online and I will post our Xcaret photos as soon as he has them up. We used a disposable water-proof camera for the river and most of those photos did not come out as it was too dark. I do have some good pics of the beach and such though.
This is probably more info than you wanted but I am a detail person. I like to know as much as possible before I try something.

Hope this helped you some! Enjoy Xcaret and have an awesome cruise!