The heavens portal is the most common thing. I would say it happens to me maybe 75% of the time. To get moat skip instead you should mash A and/or B just before hitting the wall (or time it once you know the timing). The correct timing is pressing A or B 4 frames before touching the wall IIRC. You can also get it by hitting the wall at the same time as hitting the water, but I feel like this is incredibly hard to do consistently.

Most of this I have either got from experience or from the description of the TASes so I'm not 100% sure whether this is correct, but I seem to get it somewhat consistently using this info.

You could also try this method if you are still having trouble (I have no idea whether it is consistent or not though) @4:30