The root is too thick to tie around him, but it can be thrust into the water and down the tunnel, and might be useful in helping someone pull his way back. It is soon completely dark in the underwater tunnel. Ogrin dives into the chilly water. It is cold, but not unbearable. As long as he is working hard, he shouldn't freeze. Kicking his legs and keeping his hands in front to keep from striking his head, the brave halfling does determine that there is a tunnel. He swims about 10 feet and returns. He needs light to move effectively, but clearly he could have kept going.