Airswim
Airswim (not to be confused with Frog Airswim) is an unexplained and extremely rare loading error that can occur 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. 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
Theoretically, if a player were to trigger a full-height airswim during a round of Koopa Freerunning in the Lake Kingdom, they may be able to swim through the air to take a shortcut directly to the boss arena platform where the goal is situated. This would eliminate the need to take a detour to where the Bounce Flower is located to reach the top as normal. Using mods, players have forcefully enabled airswim to get an idea of what time could theoretically be achieved, reaching a minimum time of around 14 seconds, roughly two seconds faster than the existing world record.
Sightings
The community has created a Google spreadsheet to track all recorded occurrences of the glitch.