Instructions
Open your "Roller Coaster Tycoon 3" park and navigate through the map to the location where you'd like to place the water.
Select the "Dig" tool from the toolbar and click on the ground to remove a layer of dirt. Continue removing dirt until the recessed area is the desired size and depth.
Select the "Water" tool from the toolbar and click somewhere in the recessed area you created to place water in it. Continue placing water until the hole is completely filled.
Repeat Steps 2-3 as necessary for any other spots in your park where you want to place water.