On the same day we visited Firefly Plantation, we made a stop at Old Hegg Turtle Sanctuary since they're both in the same general area (the other side of the island from where we were staying). There was a large group of cruise ship-based tourists when we first arrived but eventually it thinned out.
The facility is nothing fancy. There is one big pool where the older turtles swim, another fairly large one for the babies and a number of smaller pools where other turtles are kept. Visitors are allowed to touch the larger turtles as they swim by, but are instructed to keep hands away from the babies.
We thought the best part was seeing the baby turtles -- they are so very tiny and cute! Another highlight was seeing Old Hegg, the oldest turtle there. We were told that Old Hegg has his own pool because he was getting a little nasty with the other turtles in the large pool. I guess he's turning in to an ornery old turtle...
There is a small admission fee/donation and there's really no official tour, but the owner of the sanctuary is there to tell you about the turtles and answer any questions you may have.
There's also an opportunity to purchase t-shirts and the like.