Travel Skip is the first unintended skip in the game. It works by playing the boathouse cutscene and the 'entering Bianco' cutscene at the same time, saving around 12 seconds.It can be hard to get the hang of, but is much easier with a setup.
Clearing Goop and Timing
The timing of Travel Skip is very precise and chances are if you bonk you won't make it to the Bianco portal in time. Between each hit turn around and clear the goop on the pier to make sure no enemies spawn behind you while you're trying to waterslide.
Where to Stand
When preparing to spray the Polluted Pirhana, stand at the end of the biggest board on the pier as shown in the video above. Look at the Pirhand and angle the camera so that the goop is just below the bottom edge of the screen.
Tap Spraying is a mechanic in SMS where whenever Mario starts a new spray, water will go up farther in the air for a second before leveling out. So, if you tap R quickly you will get a constant stream of water going a bit higher than usual. This can give you extra distance to make sure you get a hit. I would recommend tapping two times.
Water can despawn when it is too far away, or if Mario is not looking at it. This is why it is important to stand where you do and why you must waterslide toward the camera until the water hits the Pirhana. If not done properly you'll have to go back and try again.
Ground Pounding in the Portal
Using a groungpound is much more efficient than a jump. With a groundpound you stay in front of the portal for a longer period of time so that as soon as it opens, Mario will go in. This also works with hovering in front of it.