Head of Civil Protection Nikos Hardalias announced on Monday evening that amateur fishing in small boats is allowed again and boats with a length of up to 7 meters can take passengers for a swim under certain conditions. However, the boats are not allowed to dock anywhere else except the port where they are registered.
The exception for amateur fishing and swimming are allowed as of May 5, 2020, Hardalias said.
According to a decision issued by the Shipping Ministry also on Monday,
“activities such as swimming in the sea,, use of private (non-motorized) marine recreation facilities, fishing from land, are allowed, while the use of motorized marine recreational boats, or those requiring the use of a motorized boat continue to be banned.”
The Shipping Ministry extended the ban on departure and arrival of private pleasure boats and professional tourist ships in Greek territory from 5. May 2020 until 18. May 2020.
The decision refers also to banning of non-residents to travel to the islands.
So far, there seems to be no decision allowing water sports again.
PS If you feel unsure, check with local port authorities.

“The Shipping Ministry extended the ban on departure and arrival of private pleasure boats and professional tourist ships in Greek territory from 5. May 2020 until 18. May 2010.”
2020 or 2021 maybe?
2020