Is the Leatherback Sea Turtle the deepest diving animal?

No, but it is the deepest diving sea turtle at about 1200ft.

The sperm whale is the deepest diving animal at about 2000ft.

"An international team studying diving capacity of two beaked whales species, Cuvier's beaked whale (Ziphius cavirostris) and Blainville's beaked whale (Mesoplodon densirostris), discovered that these species achieve the deepest and longest dives of any air-breathing species. "

Source: Softpedia