We took the Mayan Ruins excursion with our two young children and thought it was fantastic! It's not Chichen Itza but a very good choice to experience Mexican history. Our guide, Dora, was very informative. Very fairly priced. You wouldn't want to do this on your own as the ruins are about 1 hour away from the port and you wouldn't have the benefit of a guide to explain the site to you. The bus was very comfortable and the ruins impressive. We left at 8:30 and didn't get back to the port until 1:30 at which time we took a taxi to Mahahual. We sat under an umbrella, enjoyed a few Sol beers and some chicken fajitas and dipped our toes in the warm water. From where we sat, there were jet skiiers and a banana boat. I would agree that, if visiting Mahahual was all you did, you probably would be disappointed as there is not much there.
However, it is important to mention that Costa Maya has only been a port for a couple of years. Construction will boom in the next few years on the road from the port to the village. You can see all the side/offshoot roads are built and named but no buildings are there...yet. It's a shame that in 15 years, this is going to look like any other port you visit. The village of Mahahual had only 20 families living there until 2002 and now there are maybe 2000 people. It is an authentic Mexican seaside village. If you're expecting Hard Rock Cafes, McDonald's and Senor Frogs, you'd be better off visiting Cancun or Vegas. If you want to see authentic Mexico, visit Costa Maya now before it's too late. I much preferred this port to touristy Cozumel with its "One Tequila, Two Tequila, Three Tequila, Floor" T-shirts.
Jamie
All these bad reports about Costa Maya are starting to get me nervous
There is a snorkeling trip by boat too. 
