The further down, the darker it was. Eino could barely make the distinction between a grunt and a scream underwater, let about cast the Wand-Lighting Charm. Why didn't they use the Bubble-Head Charm instead? If Professor Williamson was sooooo worried about the students' safety he wouldn't have temporarily disabled their speech, rending them helpless? Be cause he was a dark wizard, and he wanted all of them dead. The only thing Eino could see, kind of, where the bubbles produced by all the kicking. Still, if he tailed the professors too closely the bubbles would obstruct his view, possibly causing him to accidentally headbutt someone. If he didn't follow them, he would get lost. The faint sound that the bubbles caused also helped him navigate back to the group in case he strayed.