How can I make hair that looks like it's flowing in water? I want to make a scene featuring a killer mermaid. I've run into two big issues: the flow of the hair and the effect of her prey's blood in the water, though I've put the blood thing into a separate question.

I want the mermaids hair to float around like it's being affected by the current, as well as move accordingly when she moves in an animation. The best examples for the hair effect that I can think of are Salazar from Pirates of the Caribbean: Dead Men Tell No Tales or the mermaid from Playdead's Inside:



How can I go about this?
First, you need to check Hair Dynamics in the particle system settings.

You can reduce gravity in the Scene settings. If you set it to a positive number, the hairs will be attracted to the top, as if they were floating.

To add a more interesting movement, you can use one (or more) force field (Shift+A / Force Field). A Turbulence should give good results ; increase its Strength, Random and Flow values. Wind and Vortex can also be used to simulate water flow.

To make the setup step easier, you can specify a relative short animation range (like 1-100), and run the simulation (Alt+A). While the simulation is running, you can tweak the values of the force fields and see the result in live.