Only three of Guernsey's beaches are specially commended in a new survey, compared with all but one of Jersey's.
Port Soif was one of three beaches recommended
The Marine Conservation Society examined water quality at bays around the UK and the Channel Islands.
The organisation believes at least twice as many of Guernsey's beaches would have received commendations if the island had secondary sewage treatment facilities.
The recommended beaches are L'Eree. Port Soif, and Grandes Rocques.
'Room for improvement'
Richard Hardy, the campaign director for Surfers Against Sewage, a group calling for full sewage treatment in the island, said he was not surprised by the results of the survey.
He said: "It's something we've been aware of for the last ten years.
"It doesn't meet the standards that we are looking for as recreational water bathers and users, there is definitely room for improvement there."