Most that I heard involved someone dying. I was in a zone that bordered that area, so while I was not assigned to a town on the lake, I was close enough that the locals knew the stories. There is even a story that passes through the missionary gossip chain that an elder died in that lake. Never been able to confirm that one. But I do know that none of the missionaries that were willing to swim in the ocean would step foot in Lake Atitlán... at least not while I was there.

Here are a couple examples from a quick google search.