At 1:30pm, Kusoldharm rescuers were called to the scene where they found the foreigner, Mr Erich Heim, passed out on the beach, with his head bleeding.
The team treated him with first aid and immediately took him to Patong Hospital, but was later transferred to Bangkok Hospital Phuket.
A witness at the scene said Mr Heim went for a swim not far from the shore, while his German wife was sunbathing on the beach.
The witness said the French tourist, identified by police as Mr Chadd Attardi, 20, who hired the jet-ski went straight into the sea and probably didn’t notice Mr Heim swimming when his jet-ski struck Mr Heim on the head.
Mr Attardi was with six friends when they left the jet-ski zone, which only allows them to stay within Patong. They ended up in Paradise Beach in the Tri Trang area, breaking the regulations.
Police are waiting for Mr Heim to make a full recovery, before they will investigate the matter.