Hey Michael,
The most efficient way to do this would be to add the Post Process Volume with its bounding box at the waters surface. You will want test the height at which the post processing is applied to fine tune the transition, sort of like a buffer zone so it is a clean and accurate effect.
This will avoid any need to add it to a blueprint and will affect any and all characters who enter the volume. Below are some images of an example I created to help you visualize the set up.
Post Process Volume Placement
The thin blue line represents my water plane and the surrounding pink box is the bounding edges of my post process volume.
Underwater Look
Here is an image of the post processing after finding some decent values within various settings. If you have any other questions, let me know.
Regards,