Fluid dynamics for the win!
Oooooh water. This was a commissioned piece for a professional water nerd. Which, if I had known in high school that water nerd was a career option, my life might have been very different.
While researching this piece I learned way more about the physics of animal locomotion than I intended. Finally I boiled it down to: squids use jet propulsion to get around. Fish have a special shape and fancy scale slime to help them overcome drag. And the special density of sea water makes the magic happen.