The only hourglass shaped lake I know of is Warm Mineral Springs in FL. It has that shape due to a sinkhole when ocean levels were lower. With a hotspring erosion setting off the process. https://en.m.wikipedia.org/wiki/Warm_Mineral_Springs_(spring)#:~:text=Warm%20Mineral%20Springs%20is%20a%20sinkhole%20formed%20in%20carbonate%20rock,metres%20(236%20ft)%20across.