Hey dead-eye, having trouble spearfishing? Find out why you'll almost never catch a fish by aiming directly for it.
The universal speed limit is the speed of light. Although the speed of light is treated as a constant, the practical speed of light is changed by the medium through which it travels. That is — in a vacuum — light can haul ass at around 300,000,000 meters per second, while traveling through denser material causes it to slow down.
How does this help a fish make you look really stupid when you're spearfishing like Tom Hanks in Castaway? Refraction. Light bends when it moves from one material to another.
Imagine you're driving a car along a road — when suddenly — some mysterious force slows down the left front wheel. The car veers to the left. If they were to slow down the right front wheel, the car would veer to the right. (At this point a diabolical voice should come on the radio and warn you that unless you play along, it will steer you into a tree. I'm pretty sure this doesn't happen to light, but I'll have to brush up on the subject to be positive.)
Back to light — the exact degree of the bend will depend on many things. First of all, it depends on the densities of the two materials. Denser materials will slow light down more. Secondly, it depends on the angle of the light coming in. Light hitting perpendicular to the surface will have no distortion – that would be like slowing both front wheels on a car the same amount. It will change the speed, but not the direction of the light. The greater the angle, the greater the change of direction.
The greater the change of the light's direction, the farther the fish actually is from where you think it is. Human beings are used to light that travels in straight lines. If you assume that light from underwater is unbent you will be 'seeing' the fish in a completely different spot than it really is.
This is how a creature with a tiny little fish brain can routinely hide in broad daylight in clear water. With experience, it's possible to adjust your perception until you can get a better sense of where fish are – for example, the frozen food aisle of a local grocery store. Just don't bring your spear in there. People get very upset.