Diving to the deeper areas may actually be counter productive and there may actually be very little life in the depths.
Any marine biologist around who can comment?
I'm not a marine biologist, but I think it makes sense that the aquatic life wouldn't be at the bottom of the lake. No plants can grow there since it's so dark. Therefore the food chain would start with the plants at the edge of the quarry.
The aquatic life that lives at the bottom would be scavengers, and any food scraps would also be found closer to the edge.