Which Mayan ruins should we visit on our cruise?
Our choices are as follows:
Tulum (Ultramar)-Cozumel
Xunantunich-Belize
Altun Ha-Belize

Altun Ha appeals to me simply b/c it is only 1 hour of travel time vs. 2 hours for the others. However, I don't want to leave thinking I didn't see much.

Which have you been to? If you have seen all/some what is the difference?

Actually:

Tulum is situated direct by the caribbean sea with a nice small beach accessible from the historical site. Nevertheless, if you are interested in mayan culture and history, Tulum is not the most important thing to see. But anyway -to start with - nice excursion.

What we loved best was a trip to the Lamanai ground (means submerged crocodile) in Belize. Most cruise (we were there with RCCL) lines offer a great thing which is a "speedboat"-trip to the historical side of Lamanai which is situated a the border of the New River in the rainforest. A belize-style lunch is served followed by a very interesting guided tour. But you have to tolerate humide climate and carry insect repellent with you.

Last but not least: Chichen Itza - is not very often on the list as it is a 3-hour drive from Playa del Carmen. Anyway, if you manage to do this, it's really worth visiting (one of the "new" world wonders).

All in all it depends on how deep you would like to get into the historical stuff.

Actually:

Tulum is situated directly by the caribbean sea with a nice small beach accessible from the historical site. Nevertheless, if you are interested in mayan culture and history, Tulum is not the most important thing to see. But anyway -to start with - nice excursion.

What we loved best was a trip to the Lamanai ground (means submerged crocodile) in Belize. Most cruise (we were there with RCCL) lines offer a great thing which is a "speedboat"-trip to the historical site of Lamanai which is situated a the border of the New River in the rainforest. A belize-style lunch is served followed by a very interesting guided tour. But you have to tolerate humide climate and carry insect repellent with you.

Last but not least: Chichen Itza - is not very often on the list as it is a 3-hour drive from Playa del Carmen. Anyway, if you manage to do this, it's really worth visiting (one of the "new" world wonders).

All in all it depends on how deep you would like to get into the historical stuff.

Tulum is in a lovely setting as tigercove said but the ships' passengers make it feel more like a theme park. I have visited Chichen Itza, Uxmal, Cobah, and a ruin outside of Belize City. Cobah which is about 40 minutes inland from Tulum is my favorite because it is underdeveloped. The last scene from Star Wars was filmed there. If you want a lovely setting and crowds, Tulum is your choice. If not pick one of the Belize sites.

Not to burst your bubble, but if you are a frequent blood donor, most CBCs won't take it for a year if you've been in some malaria areas, like Mexican ruins.
Call your CBC for details if this is important to you--I learned the hard way, and couldn't donate for a year.