A number of caving links are provided below. The Gurnee Guide is a comprehensive yet compact book that is not only sized to travel well, but contains almost 300 pages of perfectly balanced information about caves. The Gurnee Guide is a great introduction to what awaits you underground, and is a great reference too. Some caves can be surprisingly warm. Most, however, tend to be a bit on the cool side, hovering somewhere between the fifties and sixties depending upon time of year and in which part of the country.
Shirtsleeves are often fine, but a light jacket is definitely a good idea.
Most areas within show caves are reasonably well lighted. However, specific features will often be pointed out by your guide using a hand-held light. A compact, powerful flashlight can come in handy for getting a second look at those and other features. The other great thing about snorkeling is that almost anyone can afford to buy the equipment, it requires minimal training, and since the gear doesn't take up much room you can easily pack it in your luggage on trips and vacations. That's why even since I was a kid I've spent more time under the water than on top.