Depends on your view of "worth". It was for me. I'm very interested in historic civilizations, particularly the Mayans. Being able to visit Tulum, Chichen Itza and Kohunlich (also 2 hours inland from Costa Maya) which are all very different gives you such a different picture of them than just viewing one especially since it was a long-lived society and the demographics of the inhabitants was quite varied between locations. So even though I had only a very short time at each of these locations, it was more valuable to me than longer at one ruin. I also hope to see several more as part of my bucket list. So, yes, very worth it to me. But even if this is the only Mayan ruin you visit, I have to say that just standing in front of the temple was a thrill for me. It was so much more amazing than I'd expected so I wouldn't have wanted to miss that.
Also keep in mind that it's as hot as Hades even in December and there's no cover. I don't know that I could have stood up to a longer visit. With kids? That would depend on the kids. When I was young, I would have loved it and maybe handled the heat better. My sister would have hated it and complained about the heat, the distance and how it was boring.
Totally doesn't surprise me. We've been to Cozumel twice and both times, each direction to/from the mainland, at least one person rushed for the bathroom, several people sat with the sick bags at hand and many looked green. I find the movement so soothing I fall asleep and DH has to poke me when we got close to shore.