I think it would be a difficult tour for someone with poor balance. The hike in requires 3 river crossings and the walk up through the wet part of the cave (after you swim to get in) is mainly up uneven, rocky stream bed with moving water. You can use your hands on much of it and the guides are good about suggesting where to put your feet. If you start with this photo:
http://www.flickr.com/photos/staceyholeman/3885599817/in/set-72157622234749818

and keep hitting 'next', you'll get an idea of what the tour is like from the photos of our extended family group going through. Have a great trip to Belize, whatever you decide!

The balance in crossing the stream is a bigger issue, IMO; the rocks are larger than cobble-sized, and slippery. The hiking trail there is done at a brisk pace, and is fairly level, but not all of it, and it isn't what I'd call even footing.
I think that the cave itself has better footing, for the most part; it's dry in places, but not bad in the flowing water. There's a bit of a scramble to get up to the level where the pottery and skeletons are (and an optional ladder at the very back of the that level). (They tell you exactly where to put your feet and turn your body to make that scramble.)That might be challenging for someone with balance issues.
If the balance is really bad, you might want to substitute a different cave experience, even though I hate to discourage anyone from the trip (our group had oldsters who weren't in the greatest shape).

Hopeful, really nice photos! (several look just like mine, esp. Cahal Pech, which I love; just different faces!)

If you decide against it, take a look at Barton Creek Cave. You go by canoe there and are paddled by a guide. There are some relics inside like pottery and a human skull.

Great idea, volcanogirl; I was trying to remember the other option (Che Chem Ha, with tons of pottery), but I think it's more grueling a hike. (I believe that there's a new cave offering right outside San Ignacio, too, Actun Chapat.)

ANyway, here's a sample site of cave tours: http://www.cayoadventure.com/Tours/CavingExpeditions/BlueCreekCave.aspx

thanks very much everyone. Thanks for the alternate suggestions too, we will look into Barton Creek. We are spending a week at Cahal Pech so will have lots of other activities it looks like.