Airswim
Airswim (not to be confused with Frog Airswim) is an unexplained phenomenon that sometimes occurs when loading the Lake Kingdom or Seaside Kingdom. This glitch results in the kingdom's water level being higher than normal and exhibiting erratic behavior, while also causing the water to appear invisible. Using this glitch, Mario can swim in midair to reach heights that he is unable to under normal circumstances. While the exact cause of the airswim glitch is mostly unknown, it is thought to be caused by a loading error, similar to the (also unexplained) fog glitch. There is no known setup for the glitch, and instances seem to occur purely by chance.
Variants
Across various instances of the glitch, it has been noted that the height of the water during the glitch varies depending on the game version. There are two variants of the airswim glitch: the tall variant and the short variant, each occurring on different versions.
Tall airswim
Tall airswim, or big airswim, occurs on game versions 1.0.0 through 1.1.0. In this variant, the water height extends nearly to the top of the kingdom, allowing Mario to swim freely and reach numerous places that could not normally be reached.
Short airswim
Short airswim occurs on game versions 1.2.0 and onward. This variant only results in a slight increase in water height (only slightly more than the height of Mario), and does not have the same potential uses as the tall variant.
Uses
Air swim would be invaluable in Lake Kingdom Koopa Freerunning if someone were to get an air swim of the perfect height. Theoretically, one could get an airswim to the height of the boss arena platform where the goal is situated. In this situation, the player could move from the platform with the fishing pond directly to the goal instead of taking a detour to the Lake Lamode City. Using mods, players have given themselves air swim to see what time could be achieved. So far, the best air swim time is a 14 second time. Whereas the usual world record is only 16 seconds
Sightings
The community has created a Google sheet to track all recorded occurences of the glitch.