Hey and thanks for your answer! @PresumptivePanda
Yes exactly, I have also thought of luminous underwater plants. Not only as decoration but also as a design element. The player is often guided by light somewhere through the level.
I also thought about fish. I’ll definitely try to include fish that might follow a spline and speed up briefly when touching the player.
Like the sheep in Forza Horizon that always avoid you when you try to run them over 
Thanks once again,
Gazu