Changing Water Height In Game
Changing Water's Height in-game is something very easy to do, but requires some basic knowledge of the Water Properties and Water States. Make sure you have read the Water wiki.
First of all, you'll place the default Water at the place you want. You'll then edit this same Water Type, add a new State (Alternate Setting) to it, and change its Height property to something different than the Default one.
Secondly, you'll create a trigger with the Set Water State action in it. From there, you want to select the new Water State you've just created and set the other values to whatever you want.
... And voilà!