Video shot by a French environmental NGO in the Mediterranean Sea shows masks and gloves littering the seabed.
Opération mer propre (Operation clean sea) is trying to clean up the coast near the Côte d'Azur resort of Antibes.
It is appalled by this new coronavirus pollution, which adds to the already chronic problem of plastic pollution.
The group says it is sounding the alarm, as Europe slowly emerges from lockdown and people start revisiting beaches in the hot weather.
The group's founder, Laurent Lombard, who shot the underwater footage and posted it on Facebook, says "these masks - we haven't had them for long, and we're going to have billions, so I say watch out, it's the beginnings of a new type of pollution".
Diving off Golfe-Juan, near Antibes, he found five disposable face masks and four latex gloves on the seabed, along with the usual plastic rubbish such as empty bottles.
Already on 14 May - just three days after the French lockdown was eased - street cleaners in Paris complained on social media that many masks had been discarded on the capital's pavements.
An MP in the southern Alpes-Maritimes district, Eric Pauget, has put forward a draft law to increase the fine for littering a public place with masks. It would fine an offender €300 (£265; $330) instead of the current €68, news website LCI reports.