Surprise! It's corpses!!
I loved hearing everyone's guesses as to what could be under the lake! There were some really creative answers (and some goofy ones that gave me a laugh.) But I had actually told you guys what the lake was made of. To refresh everyone's memory, we heard back in the extra comic of Chapter 5 about Helene's father, who lodges the souls of the wicked in the Death River Nevah. As it says on the "No Swimming" sign, this lake happens to be fed by that river - which is why these accursed corpses are trapped at the bottom of it.
The reader who got the closest was dillonmetzger! Dillon guessed that whatever was under the lake was "...locked up down below the water either as punishment for something..." and these wicked souls are trapped below the water as a punishment. Congrats Dillon! You get a colored digital sketch from me. Let me know what you'd like me to draw for you in the comments!