There are multiple ways to trigger this glitch:

• Sprint swimming up a vertical water stream caused by waterlogged slabs results in the player being able to maintain the swimming animation outside of water. As long as the player has blocks above them, you can continue to swim forward while pressing the spacebar.
• Sprint swimming into an area with signs on top and a block dug down at the bottom. You can continue to swim forward through the "tunnel" even though there is no water present