That divergence happened a long time ago, fish and amphibians specialized for their particular niches, mammals, birds and reptiles specialized for theirs. All the creatures alive today have had the same length of time for evolving. A smallmouth bass is every bit as beautifully adapted to its niche as a western gray squirrel.
Fish have to be streamlined to move with any speed.
The viscosity also creates feeding problems since the food has to be separated from the liquid. Water dissolves relatively little oxygen. Water can hold up to 20 ppm oxygen. Fish have to have an extremely efficient respiratory system. Details about fish respiration Because of differences in the medium, the sensory system of a fish differs from our own.