According to Koh Tao Tourist Police, Hanspeter Suter, 44, had just completed a Dive Master scuba course on the tourist resort island.
Three days before he went missing, he was told by a doctor that he should not swim because it could lead to blindness.
Police said Mr Suter was under stress and did not have any money.
He rented snorkelling gear from a shop on the beach late Saturday. He left his passport and personal items there, but did not take a life jacket with him. He did not return to the shop, it was reported.
Sairee Beach is the same area where two British tourists, Hannah Witheridge and David Miller, were murdered in September.