Sorry, nope, no idea. I checked your file and you did all that is needed to have a mesh with armature interacting with a fluid simulation… but, for some reason, Blender only acknowledges your armature in rest position.
I made a quickie. A mesh with 2 bones which comes in and out of a cube of fluid. It works, whether the animated mesh is fully inside the fluid domain or not. (There’s a myth floating around which says that it matters.) Your scene just doesn’t work.
Applying scale to everything (NOT to the armature!) makes no difference. Changing the time of the simulation to use real time instead of slow motion, makes no difference. A bigger fluid block makes no difference.
I found nothing weird like triple unused modifiers, mind-bending shape keys, drunk drivers to take Blender off the road… You can hardly call weird a few bone constraints with no effect. So…
What did you doooooo?