Question on Florida Key West?

My main question is how are the beaches? Are they nice turquoise and clear water like the tropics or dull and black like new yorks? Is the sand White? Is there like areas of the beach that is around 6-8 feet deep cause i like water thats not too deep but deep enough were i cant stand (MUST BE CLEAR WATER) and dangerous animal life in the beaches of Key west? Also is there scuba diving for teens from 15-18?