Once you have drained the pool do a bit of backtracking and you will find another room with a second valve. Turn it to fill the room with water then jump in. swim down halfway to an underwater tunnel.
Swim into the tunnel and follow it (taking a breath at a midway point if you need to). At the end, surface. Kill any Draks and get out of the water. Move in and take the crystal. Now, with all three crystals, go to the transporter room. Place the three crystals and watch as the time warp opens. Jump in and get ready to go to the Old West!