Free diving is diving without scuba. Skin diving means diving without a wetsuit or scuba. Since you can free dive without a wetsuit they can be the same thing but they are not necessarily always the same thing.

John L. Cramer has written:

'Skin and scuba diving' -- subject(s): Skin diving

Kurt Svrcula has written:

'Diving in Malaysia' -- subject(s): Deep diving, Guidebooks, Scuba diving, Skin diving

George F. Brookes has written:

'Underwater swimming' -- subject(s): Scuba diving, Skin diving

'Sub aqua' -- subject(s): Skin diving

Steve Rosenberg has written:

'Diving & snorkeling, Monterey Peninsula & Northern California' -- subject(s): Guidebooks, Scuba diving, Skin diving

'Diving and snorkeling guide to northern California and the Monterey Peninsula' -- subject(s): Guidebooks, Scuba diving, Skin diving

'Lonely Planet Diving & Snorkeling Turks & Caicos (Lonely Planet Diving and Snorkeling Turks and Caicos)'

'Diving Cozumel' -- subject(s): Guidebooks, Scuba diving