There could be two reasons:
1) Straight line distance normal to the surface is the shortest distance.
2) There is refraction of light when viewed from an angle to the normal of the water surface. The view of the fish as seen from outside the waters, is not correct. The fish appear to be a height less deeper than actual. But when viewed from the top, normal to the water surface, the fish are in the same direction as they appear.
So the kingfisher dives in the right direction when jumps in vertically normal to water surface.