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
A friend

1. Get your gun in your hand and make sure its not raised
2. run into the ocean until you start swimming WITH the gun in your hands.
3. 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.