Might help if you're a bit more specific about what isn't coming out right, or some screen shots. Basically there are a couple different ways to tackle a lake. I prefer to use a single terrain for the water, and then I sculpt the surrounding banks with more terrains. I've also seen some good results with a single terrain with the middle "dug out", combinded with a water plane. Lets see some screen shots and I'm sure you'll get alot of help here.