Step 1: Find It
Find a spot with at least two block deep water.
Step 2: Dig
Dig a couple blocks down.
As you can see, I dug 4 blocks down in the deepest part of the pool.
Step 4: Ladders
Add some ladders, they will stop the water and make for an easy escape.
Step 5: Expand
Now expand and do whatever you want in it!