Swim without having the flippers.
It can be combined with Walk on Water.
Exclusive to version JP 1.0 only.
How to perform it
The glitch is performed by lining up Link exactly one pixel from a screen transition, jumping straight into the water, and then immediately transitioning the frame after you hit the water.
One thing to watch out for when doing this glitch is getting hit in the water will trigger the game to check if you have flippers, then try to place you out of the water exactly where you last jumped in. If you entered from a different screen, this will cause an infinite scroll soft-lock.
The game allows Link to move for 8 frames in the water before it sends him out for not having flippers. In later versions of the game, this instruction exists in Bank 07:
LDA.b #$02 : STA $5D
This sets Link to the recoil state, the same state he's in when taking damage from an enemy. When in this state, Link cannot move. Since this instruction is not present in JP1.0, Link can move. It also just so happens that 8 frames is exactly how long you need to build up enough speed to start moving right. This means that despite the glitch technically having a larger window, the actual abuse of it is frame perfect. When moving west, the faster speed gain in this direction gives 1 more frame of leniency.
The best use for this is to get to the whirlpools that can be used to quickly warp to a different location on the map.
Desert Palace to Death Mountain
The most common application to use fake flippers is in the Any% (NMG, No S+Q) category when traveling from Desert Palace to Death Mountain.
Here is a picture demonstrating how to easily line up the glitch.
Use the wall (green line) to line up Link perfectly with the bush, then once touching the bush, slash against it and hold Quick Hop into the water (orange line) and hold right while in the air to transition the screen.to hold your sword out. Use the sword slash to buffer a direction change to (blue line). This will line Link up perfectly (one pixel) from the edge of the screen.