I spent 5 minutes sticking my face in the Water Ball for the Ocean Descent Demo. You can hear in the headphones like you are sticking your head under water. Awesome! I also noticed if you swing your controller through it, the water splash follows your controller. The point being, it reacted the way I thought it would.
From what i can tell, if you breathe out audibly (blow) when your head is in the waterball, bubbles come out of your "mouth". I also just like holding the controller against the luge ball and seeing/feeling the grind. oh and hitting the bullet shells for london heist. and if you wait long enough, slow mo bullets start flying at you and you can dodge and deflect them. those menus are all great fun.