A pond of water appears less deep due to –

The apparent depth will look less that its real depth due to the refraction of light. First of all, imagine an object at the bottom of the pond, emitting three beams of light: one straight to the centre of your eye, one above your eye, and one below. Now, as light travels faster in air than it does in water, it will accelerate as it breaks the surface, at which point it bends away from the 'normal' - the imaginary line perpendicular to the surface. So back to those three beams, the one coming straight at your eye, and as such perfectly vertical, will not bend one way or another as it leaves the water.