The water cools down as you make your way downstream, but remains comfortable. After several minutes you find the water drops down into a tunnel. You can't see where it goes, but hear a distant roar indicating a waterfall, which suggests and even larger tunnel. Excitedly, you pull out your journal and make more notes. A large cavern would be a fantastic find!
As you start to think about a place to attach some ropes and attempt a descent, you hear a hissing sound. It's very close. Turning your head, you catch sight of something move toward you very fast, and very scaly. In a panic you jump back, realizing too late that there is nothing to jump back too. You hands and feet stretch out to try and find purchase, but the rocks are too wet, and you drop into the hole.
For a few panicky moments, you're under water and sliding out of control. Rolling over you come can come up for air but can see very little. A low rock knocks your lamp off your head. As you grab it, you kick out again with your legs to try and stop yourself, but the current is too strong, you're going too fast, and everything is too wet.
A gust of wind blasts past your face as the tunnel ends and you find yourself in a free fall for several moments. It feels like an eternity, but just as quickly you hit the water. The current is still, and the water is cold. Kicking up to the surface, you point your lamp around quickly. There is a dry spot fifteen yards away, which you swim too and pull yourself out of the water. Sitting for a moment, to let the adrenaline pass, you check for injuries. Aside from a few bruises, you managed to survive that okay, but now you're in a larger cavern, and the way you got in is not going to be the way out. Your also very cold and tired.
You aren't sure what it was that you saw. It looked like a large gila monster, but it moved very fast. That being said, you're pretty confident you got away. It would have to be pretty dumb to follow you down that passage.
You're just going to get wet again, so go ahead and explore.