depends how much money you want to spend and how much water you want to waste .
most 6 ft. Nicer ones have 8 or up to 10 ft. deep ends.
It is impossible to know, you need to know the third dimension...unless 12 or 36 are the depths; in that case, is one deep pool!
That would depend on what you want to do with your pool. If you want to use it for diving then you will need a minimum of 8 feet (I would want 9 or 10 feet as you will hit bottom if you just jump in...
because of refraction of light same principle as when you look at a straw in water and it looks broken