The dark Lake and the breathing Underwater

"I dreamed I was swimming in a lake with a group of people I didn't really recognize, though I felt like I knew them. The water was clear near the shore, but farther out it turned this dark blue-black, and I remember thinking I could still see the bottom even though I obviously couldn't. Someone dared me to swim out to a floating platform. I said no at first, then did it anyway. Halfway there my legs suddenly felt heavy, like the water was pulling me down instead of holding me up. I tried to yell but only got out half a word. Then I was underwater, but I could breathe somehow. It was quiet down there, almost peaceful, and I could see lights far below me like windows in a building. I wanted to swim toward them even though I knew I shouldn't. I woke up right as I started moving downward, and my chest felt tight for a few seconds."