Just as the underdark is huge, it should be evident just how big this lake is starting to get and in truth we have yet to really even cover it. While maps like these may look to be void of any possible encounter or something for the players to do it couldn't be further from the truth. Maps like these give the dungeon master a chance to do something different, perhaps a underwater encounter like fiendish sharks or something flying above the water looking for an easy meal. You could even have the party ambushed while in a boat by slavers or drow (or both) from advantage points of clinging to rocks coming out of the water surface.