The woman was just a few minutes into the dive yesterday when she signalled to her instructor that she was ascending from a depth of less than two metres, the Daily Mail has reported.
At the surface, as the instructor tried to help her, she coughed severely before losing consciousness about 10m from the shore in Cala Martina cove, on the eastern side of the island.
The instructor towed her to the beach where he and two colleagues tried to revive her before an ambulance arrived. Paramedics managed to stabilise her for transfer to hospital.
Following her admission, a health service spokeswoman told press: “A 45-year-old British woman is in a very serious condition in the intensive care ward with symptoms of drowning.
“She has a pulmonary oedema, or liquid on the lungs, following a diving accident.”
Staff at the dive school from which the woman was diving told press that she had filled in the standard pre-dive questionnaire, in which she had not listed any pre-existing medical problems.