If you ever swim in lakes, you will know how the river feels. Cold at first but after you get wet, it's fine. It will be 80+ degrees when you are there, so don't worry. Today, it's 84 in Belize.
The caves aren't real long, they are quite high and probably 30' wide. There is a while where you cannot see light at either end of the cave. But, you will have your headlamp as will everyone else so it's not too dark.