What do you call the video effect where water looks like it's standing still when filmed
This is called stroboscopic effect.  It refers to when the sample rate is synchronized or very slightly out of sync resulting in a much slower representation of the motion that results from taking a picture that is at the same point or slightly advanced point in a subsequent cycle of a higher frequency cyclic motion.

You can actually do the same exact thing in real life for your own eyes by using a strobe light, which is the more common demonstration, though frame rate can be used too if you can adjust it closely enough.