Sunken Ship Clip
Normally, to enter the Sunken Ship, one must obtain the Holy Eagle (double jump) dojo technique, then use Crescent from atop the Moon Turret near Ankoku Temple in Ryoshima Coast. This parts the sea around the ship, allowing you to reach the entrance. We can skip this by clipping into the ship's geometry and hitting the loading zone from behind.
First, you must reach the ship. To do this without Water Lily (for NG Any% with Fire Tablet), you must jump with gold dash from the point on the shore closest to the ship. This point is roughly North East of the dojo and North or North West of the ship. Double jump just before Ammy hits the water, and do not air tackle, as this loses Ammy's momentum going into the water. Mash jump while swimming toward the side of the ship, where there is a platform that goes slightly below the water. Using the outer camera mode here helps take a straight line, reducing the chance of drowning.
See this image of the platform you should swim toward: https://i.imgur.com/41fqc2e.png.
Once on the ship, head toward the beam to the right, which juts out just above the water. Swim into the corner formed between that beam and the hull of the ship. Mash jump while wiggling side to side until Ammy clips into the ship.
See this image of the beam where you can clip into the ship: https://i.imgur.com/xdrRiDj.png.
You may clip fully into the ship, in which case you should immediately turn left and swim toward the glowing hole in the ship. If you clip only partially into the ship, carefully adjust Ammy's angle so she is nearly parallel to the hull, swimming in the direction of the hole, then mash jump until she clips fully through. If Ammy clips all the way across the ship, such that she remains in-bounds, swim back around toward the small ramp next to the beam to get an air refill.
Water Lily Variant
In categories which obtain Water Lily, a more consistent method of clipping into the ship can be performed. Place a water lily in the same corner between the beam and ship described above. Then use Galestorm to push the lily pad into the corner at a roughly 45 degree angle, and simultaneously walk into the corner while the it drifts under Ammy. Ammy should walk right inside the ship.
Water Tablet Variant
If the Water Tablet is equipped, one can simply dash into the corner at full speed to clip into the ship extremely easily.