You really dont need a tour to do Xunantunich...you can take the bus about 30 minutes with stops etc. Take the pull ferry accross the river and walk up the hill about 15 minutes to the entrance..the site has several museum like buildings that will explain the ruins history etc etc. After you can walk down the HWY back toward San Ignacio about 10 minutes and go to the Trek Stop, its a butterfly farm, I did not go for years as I thought butterfly farms would not interest me but I was wrong, it was very interesting...this would be a full and inexpensive day. Catch the bus back outside the Trek Stop, they run every 30 minutes.
The ATM tour is by far the BEST!!!! It has been documented in National Geographic. it is more than incredible,,the water should not be an issue the only time you have to deal with the water is when you enter the cave you have to wade across, other than that there is never any danger of getting wet. This tour is under discussion of being closed down as there are sooo..many Mayan artifacts, humane remains etc etc that the site over time is in danger of being deteriorated due to the amount of people going through,,it is a MUST SEE!!!!!!!