Hey ! 
Thank you Neongho, yeah I can’t get the overlap between my camera and my volume, I tried to see if I had any result with a print string but can’t get it to work…!
Hello pezzott1 ! Thank you too !
Honestly I tried things like that but ended up in a big brain fart since then…
I’m not home and with a laptop right now so I don’t have my project but I’ll try to recreate something similar so I can show you the idea…!
This is my character and the camera (child of the head bone)
Here I have a simple plan with a water material applied on.
Under that I have a Physics volume to simulate a “water” physic on my character
And I have my post process volume (same size and place as the PhysicsVolume) to make a blue effect (in the end I’ll use a Chameleon Post process effect)
For exemple I tried to have a trigger box with an OnActorBeginOverlap triggering the post process effect but it ended up playing the effect when my character’s feet or other parts were touching the box. (So I tried to trigger it when the camera or the head bone are overlaping the box but I’m a big blueprint noob I’m so sorry…)
Same thing for the movement mode “Swimming”, I don’t want to trigger the post process effect if my character swims but my head (camera) is above the water surface…
I have the animations working, I have the Physics working, but I’d like to know if my camera is underwater or not, so I can apply a low pass filter on my sounds / call particles effects and post process effect when i’m going in and out the water, etc…
I’m so sorry, thanks you two for your time and your kindness !
Maybe there are some really easy solutions to this, but I still don’t have the logic… (but I love doing blueprints tho).
I’m a sound designer / composer but I’d like to be able to program all my sound effects by myself… So I’m learning ! 
Have a beautiful evening
Lyptus
)
