These terms have changed their meaning over the half-century or so since scuba and breath-hold diving became popular. In the 1950s, "skin diving" (American English), free diving (English translation of the French "plongée libre") and "underwater swimming" (British English) meant both scuba and breath-hold diving. The term "skin diving" was called "skin diving" to distinguish it from "hard-hat diving", where the diver dressed in a canvas suit, a copper helmet and lead-soled boots. "Skin divers" wore either just swimming trunks, fins, masks and snorkels or perhaps a thin drysuit or wetsuit. "Free divers" in the 1950s got their name from the fact that they were "free" to swim around underwater, unlike hard-hat divers who walked on the seabed. "Underwater swimmers" again contrasted with hard-hat divers by virtue of their freedom to swim about underwater.

Nowadays, "skin diving" is an obsolescent term, although some people want to make it a half-way house between free diving and snorkelling. "Freediving" now means diving down under water, sometimes to great depths, on a single breath. There are strategies that modern freedivers learn to extend their stay underwater while holding their breath. "Underwater swimming" is a slightly old-fashioned term nowadays and can mean simply what a lapswimmer does after doing a tumble-turn and before returning to the surface.

Now the two terms scuba diving and snorkelling. These are much clearer terms because they focus on the swimmer/diver's breathing. "Scuba" is an acronym standing for "self-contained underwater breathing apparatus". Scuba diving is therefore any kind of underwater diving with air tanks. There are many different scuba diving disciplines, from cave to wreck diving. "Snorkelling" means swimming or diving using a snorkel or breathing tube. Although a snorkel allows the swimmer or diver to look downwards in the water without having to turn their heads to breathe, the snorkel is quite a short device so a snorkeller diving down underwater won't be able to replenish the air because they are far below the surface of the water. Incidentally, snorkelling is often, wrongly, dismissed by some scuba and free divers as a tourist pastime requiring no skill whatsoever and involving nothing more than floating on the surface in a lifejacket. I have snorkelled for over fifty years and this image of snorkelling is far removed from my own experience. There are now versions of snorkelling focusing on distance swimming, such as river snorkelling (the snorkelling equivalent of white-water swimming) or the activity of swimtrekking, involving open-water swimming from one Mediterranean island to another using fins, masks and snorkels. It's great to be able to combine a love of walking on land with the opportunity to snorkel across lakes, rivers and seas while traversing the country.

You ask about equipment. Masks will be pretty much the same across all the above water sports, although there is a tendency nowadays to push "low-volume" masks because of their ease of clearing for scuba and free divers. This is not so important for on- and near-surface swimmers. I like my high-volume mask when I snorkel, but that's a matter of personal preference. What does matter is that masks should fit faces without leaking, which is why they should be tried on before purchase. As for fins, this can be a minefield as everybody has their favourite fins and believes that everybody else should be wearing them too, which of course is nonsense. The best fins in the world are those that fit the purchaser and suit their power, manoeuvrability and endurance swimming skills. They don't have to suit anybody else. Scuba divers tend to prefer open-heel fins, such as Scubapro Jet Fins, particularly in colder climes when suits are worn. Some scuba divers in tropical areas prefer full-foot fins. Free divers tend to prefer long-bladed closed-heel fins, which give them the power and momentum to steer their way far below the waves. Spearo fins resemble freedivers' long-bladed closed-heel fins, except that they often have camouflage colours. As for snorkelling fins, the great thing is that you can wear pretty well any kind of fin. When I snorkelled at La Jolla Cove a few years ago, the snorkellers were wearing open fixed heel bodyboarding fins, all-rubber full-foot fins, composite fins with plastic blades and thermoplastic foot pockets, adjustable open-heel fins, long-bladed freediving and spearo fins. So when it comes to snorkelling fins, the focus should be on choice. Some diving store retailers think, wrongly, that all snorkellers are wannabe scuba divers or free divers. Those of us who enjoy adventure swimming with masks, fins and snorkels prefer to make up our minds when it comes to fins, not to have our minds made up for us by salesmen who have never snorkelled other than in a pool or shallow lake.