Previously, from Battlegroudns Mobile, your character died immediately when the character got knocked out from the water.

After version 1.8.0 update, however, your character can still swim after getting knocked out instead of facing immediate death.

Although your swimming speed slows down, you can still swim while you are DBNO’ed.

You can reach out to your teammate for help even if you’re inside the water. So try to stay alive until you get help from your teammate.