Diving
Diving is jumping or falling into water. It can also mean swimming beneath the surface of the water.
Kristin was injured twice while diving off the Cliffs of Heaven in a recreation of Cirrus IV on the holodeck. (TNG: "Conundrum")
Skindiving
Skindiving is diving without a wetsuit, using a snorkel. Once, Geordi LaForge 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 lifeforms. (TNG: "The Loss")
Scuba diving
Scuba diving is diving with special self-contained breathing equipment.
Scotty used a force field instead of a normal wetsuit and breathing aparatus when diving on planet Argo.
Other References
Sky diving
Sky diving is the falling part without the water part.
See Orbital skydiving.