Well i was gonna suggest a few things but looks like you’ve already done them all. 1: A wide FOV will help the effect (already done) 2: Use a particle effect for bubbles i think. and 3: You’ve already got caustics as well 
Well i was gonna suggest a few things but looks like you’ve already done them all. 1: A wide FOV will help the effect (already done) 2: Use a particle effect for bubbles i think. and 3: You’ve already got caustics as well