Do this after you have drained the water from the town. This technique is extremely difficult, even harder than getting to the top of the castle early. Enter Wet-Dry World with the water as high as possible. Now, jump into the pipe that leads to the town. Once you come out the other end, look left and find the grey building with the flat roof. Drain the water. Make your way on top of this roof and look back towards the pipe. Take a run up and long jump onto the red roof below the pipe entrance, aiming for the fire-spitter. With luck, the fire spitter will light you on fire. When this happens, run in a circle so you end up facing the way you started (to my knowledge this is the only way to build up enough speed). You should start sliding on your butt towards the brown pillar on the wall. After a split second of sliding, jump towards the pillar, wall-kick and hold right. With a large amount of luck, you'll grab the top of the grating. From here, you can pull yourself up and drop down back into the pipe.
This has a couple of weird effects, number one, the camera can't handle being in the pipe with no water so it forces itself to stay outside, and when you get halfway down the pipe you pop to the water's surface in the main area.
Submitted by: Dom Dunc