For stingray city trips I recommend going out on the Heavenly Hooker. This is a great boat that was refurbished after the hurricane a few years ago. It is also never packed full like some of the other operators. Ezona and Stacy (Husband and Wife operator) are also very knowledgable and friendly.
scubagruba- Don't worry about Reggie.(Assuming you meant Reggie in Belize) I was a little leary too, but they are very honest people. Just make sure you bring your confirmation email and make sure you check your booking date. If you read my review you will know why. But even though we had a miscommunication on the date, Reggie went well out of his way to make sure we got our excursion.
Scubagruba- They had a variety of tube sizes to small to extra large, so you shouldn't have a problem with the tube being too big. They also had different sized life vests, but Reggie supplied those, so you might want to email him and tell them what sizes you may need. Or just tell him how many of what ages you have in your group so they bring the appropriate size vests. The tubes are supplied by the national park where you start. Bring water socks. Its about a mile trek from the parking to the drop in point and without shoes, it will be a problem. The water socks can easily be worn in the river too.
For time zones... I think GC works on eastern time but I'm not sure. The time zone of your embarkation port is normally the time you stick with and they call it "ship time". I say normally because NCL changed time zones when we went to Belize and then changed back the next day.. Not sure why.