This is a very simple technique for creating realistic lakes and Ponds. In order to re-create this technique you will either need smoncrie's hole digger lots or you can use the Mayor Mode ground lowering tool. You will also need jeronij's ploppable water, available on the STEX.
Lay "sunken" hole digger lots in a random shape to your liking. When doing so, remember that you will still need to draw a road over them to activate the lots, so be sure you are able to do so with aligned lots.
Demolish the digger lots and then define the boundaries of your pond/lake using single road tiles.
Next, create the banks of the pond/lake by covering all sloping sides using a grass filler tile of your choice (I have used the in-game grass lot but there are many variations available on the STEX).
Create a road around the lake (optional of course) and fill in any empty tiles with your grass filler lots.
Now add the ploppable water...if you really want realism then do not be afraid to overlap your grass filler lots slightly..Ponds and lakes have very few straight edges.
Finally, add trees to a) help disguise some of the rougher edges of the water b) add realism..where there is inland water, there are generally trees & c) because it looks nice!