I have found a number of diving sites that use < plongée libre >, as well as < plongée en apnée >. Decathlon uses both in its advertising, and Collins also translates 'free-diving' to < plongée libre >.
And for snorkelling - Larousse gives < faire de la plongée avec un tuba >, which seems to be about the only expression that gets the red line. (A mask is not essential equipment for snorkelling)
https://www.larousse.fr/dictionnaires/anglais-francais/snorkelling/612936
