While holding a knife (kitchen knife) while swimming, your character won't actually be holding it anymore.
The knife is floating just out of the reach of the fingertips, following the movement of the hand.
This is also during treading in the water. The knife is no longer in the hand, the hand is open and the knife is attached with an invisible sting to the fingertips.

Option to solve: Leave the knife in the hand, closed, OR, Once you actually start swimming, make your character put the knife back in inventory.