This is a glitch that plays the swimming animation globally while not playing local thus allowing you to function normally while people see you as swimming.
To swim glitch you will need:
A primary gun
- Get your gun in your hand and make sure its not raised
- run into the ocean until you start swimming WITH the gun in your hands.
- swim around for a couple seconds, then come back, and if your friend says he sees you swimming on land its working.
Note: You cannot see yourself swimming.