Simply put, I wanted to swim in the air, so I ran this inside the "water.json" file, located in datapack/(namespace)/data/minecraft/tags/fluids:

{ { "replace": "true", "values": [ "minecraft:empty" ] }

When running the datapack, water turns into air physics and air turns into water physics as intended. I can hold the jump button to go up and the shift button to go down, flying through the air with the water fog and everything.

However, when try to sprint-swim, it bugs out a bit, only allowing me to sprint-swim at spontaneous times.

Also, I can't drowned in it. The bubbles that appear above the food bar flicker into existence and back out at spontaneous times.

This only leads me to believe that there is an issue where it flickers back and forth between water physics and air physics.

Using "minecraft:lava" and "minecraft:flowing_lava" in the "water.json" file works as intended, with sprint-swimming in lava completely untouched and drowning fully functional, so this only applies to air, or "minecraft:empty".

(In the lava picture, I have Resistance V on, so I'm not taking damage.)