I'll second what Snoozeman stated...it's a nice little park.
I went there as part of an island tour through the ship. If one hasn't been to a Mayan site, I think it's worth a see for sure. It's small compared to some other sites, but has its own charm. To me it looks an Indiana Jones movie with the jungle growing into the ruins somehow.
For the record, I've also visited Tulum, Uxmal, Kohunlich, and Xunantunich Mayan sites, but still consider San Gervasio to have its merit.
Next year is Chichen Itza....so I guess I love anything Mayan.