Well, now cold and hungry you're starting to think that the day couldn't get much worse. You are now in total darkness, looking up there is a tiny light where you fell through. You call up to see if anyone is there, if anyone saw you fall and can help. You get no response. There's no chance of you climbing back up there so you start to swim forwards; you have no idea what direction you're going in but you know that you need to get out of the water quickly or you will freeze.
After what feels like an hour of swimming you collide rather suddenly with a wall. Treading water, a little dazed, you investigate the wall. It appears to be made of some sort of rough rock which verifies your thought that you've fallen into a cave of some kind. You feel your way along the wall for a while, with no plan in mind.
You are starting to shiver and close to giving up when the surface of the wall changes. It now feels smooth, like marble, and carries on sideways for about a foot. It also seems to carry on upwards, as far as you can tell anyway. You feel your way down into the water, the marble carries on downwards too.
This is the first sign that there might be a way out of the water. Diving down would mean getting much much colder but could get you out of the darkness. You could carry on swimming round the cave to see if there is anything more there at the risk of freezing to death. Or you could try climbing the cave wall next to the marble to see if there is an opening near the top. All three seem dangerous in your current half frozen state, but you have to choose one.