While in the process of moving from walking to swimming the animation spawns you under the ground underwater with just your head showing. then after a second it corrects itself to place you in the water. Also if you have equipment on your back e.g. gun and backback it glitches and flashes as if you are dropiing it on top of the water repeatedly. (You don't lose anything just the animation isn't working correctly.) as soon as you leave the water it goes back to normal.