Devala Lake was a large body of water.
At some point prior to 2370, Geordi La Forge and Data went sailing on Devala Lake. Data decided to go swimming, jumped out of the boat and sank straight to the bottom. Because of his lack of buoyancy Data was forced to walk 1.46 kilometers along the bottom of the lake to get back to the surface. It took almost two weeks to get the water out of his servos. (TNG: "Descent, Part II")