This can be done on the World War One mission "Germans on Run". There is a river you will come across during one of your objectives - right near the end of the level. Run into the water and keep walking in the water. You will find a spot where you can completely submerge in the water. Right after you die it will unlock.
Additionally, during the objective "Keep up with Petrovich" you will come come across a mill. There is a small river that runs alongside of the mill. At the base of the water wheel there is a spot for you to completely emerge yourself under water.