A footer that is a concrete floor lit edge to edge by water light from a ceiling you cannot see, with rounded top corners. Sharks pass through it, in every direction, and there are no sharks, only their shadows. They swim like real animals: they can only move along their heading. A resting pointer is noticed and avoided or inspected, a click makes them bolt away smoothly, and hovering a link slows the nearest one. Switch to butterfly koi if you prefer.