Ok, I can see that you used an infinite plane for your water(only use them for oceans). delete that - and replace it with a CUBE or cylinder from create up top rescaled and moved to fit in where you want your lake with the water texture of your choice. You might want to save the texture for your infinite plane before you delete it. select the plane, enter EDIT menu at the TOP (the one next to file NOT the one next to create). In the edit menu there's an option to save it - remember to load the texture into your cube using the same menu. Chances are you'll have to adjust the postion and size of the water "cube" a couple of times so it looks right.