Here is another way of thinking abt it; i pretty much made it up right now but it might help understand it better, p.s this is not to be taken too seriously, but it's an explanation that should suffice for now...
Think of it this way, light has a certain amt of energy given by E = hf. Right, now water is one medium and air is another and they are both horizontal, like a pool water on the bottom and air on the top, water has surface tension,
now light wanting to get out of it will have to go upward, so resolve vectorally, cos90 being zero and cos zero being 1... Therefore the light will have the greatest y-axis component when the light is at 90 and the least when light is at 0 i.e parallel to the surface of water... Therefore light will have least amt of problem getting through the surface tension of water at 0 degrees and the maximum problem of getting out of it 90 degrees... Therefore depending on the refractive indeces or the diffrence in pressure at the join of the two surfaces, we have a critical angle...
I hope this makes some sense...
Guyz please go easy on me...