Skindiving is diving without a wetsuit, using a snorkel. Once, Geordi La Forge went skindiving off the coral reef on Bracas V. He would later relate this experience when the Enterprise was being pulled through space by what appeared to be a school of two-dimensional beings. (TNG: "The Loss")
Scuba diving is diving with special self-contained breathing equipment.
Charles Tucker III was an experienced scuba diver. He kept a picture of himself scuba diving in his crew quarters on Enterprise NX-01, as well as an antique diving helmet. (ENT: "These Are the Voyages...")
In 2365, a holoprogram available to crewmembers of the USS Enterprise-D was "Scuba Diving Hanauma Bay Earth". The program, with a code list number of 03-889, was listed in the exercise menu. (TNG: "The Emissary", okudagram)