This is pretty tricky. There doesn’t seem to be a textbook reference that clearly defines Skin Diving, but I will do my best to define it and explain what makes it different from Freediving. This is based on my own understanding, coming from two skin diving (not freediving) organizations. I’m an avid skin diver and only called myself a freediver after getting certified. After experiencing the course it was even more apparent to me what the differences were. There seems to be a general confusion I hope to help clear out.
Freediving is generally diving down with only one breath. What makes it confusing is that it has many applications: Spearfishing, muro-ami, manual aquarium fishing, skin diving (sometimes still even referred to as snorkeling), and I’d even argue underwater hockey to a degree, among others.
Skin Diving is basically advanced snorkeling in a sense that you also just do it to enjoy marine life, but beneath the water’s surface. Skin divers apply some practices in freediving to do the activity safely. This is probably why some refer to it as recreational freediving.
I’m showing two photos that seem to show the major differences between Freediving and Skin Diving.
Objectives: Freedivers dive for depth, many doing it competitively or as a sport. Skin divers dive for fun or to enjoy marine life up close.
Venue: Freedivers would typically choose a deep spot where they can drop a line many meters deep, oftentimes not seeing the ocean floor. Skin divers usually pick an attractive coral reef or any other spot with a good assemblage of marine life.
Depth: Certified Freedivers could dive from at least 16 meters to over a hundred. Skin divers don’t need to go that deep because coral reefs thrive in the shallows, so the average skin diver would typically just hang around 2-10 meters.
Body orientation: Freedivers’ bodies are usually seen oriented vertically. Skin divers’ bodies are usually seen oriented horizontally.
Fins (NOT strict. This varies, but usually): Freedivers could wear long fins which allows them to go deeper more efficiently. Skin divers would be fine with regular snorkeling fins. They don’t need to go that deep to enjoy marine life. It’s also more difficult not to hit corals with long fins in skin diving, so short fins would already do for them.
Other equipment: Dive line, lanyards, weights, freediving wetsuits, freediving watches, noseclips – these are more commonly used in freediving. Skin divers could do well with basic snorkeling gear: mask, snorkels, and fins.
I hope this helps.