I can't think of any tutorials for that specifically...
Anyway, the terrain(s), or objects, that is going to 'cut away' should be set to negative ('A' - Attributes). The object(s) that it is going to be 'cut away' from should be set to positive.
Then group ('G') the two.
 - A material on the negative terrain will cover the cut-out part. Use a rocky material, or whatever, for the water bed.
 - Some part of the negative boolean terrain has to be above the postive boolean part, or you can't see it.
 - You can select, move, and apply materials after the boolean. CTRL + mouse click, select from the list that pops up.
Then you need either a plane, a terrain, or an infinite plane' to use as the water. Doesn't have to be booleaned. Has to be somewhat below the 'lip' of the cut out. I usually use terrains for water, with a 'wave' image applied in the terrain editor, to get some waviness for the surface.

Friends don't let friends use booleans.