The 76-year-old woman was swimming off Michaelmas Cay, a sand island near Cairns, when she was pulled from the water before 2pm on Tuesday.
Tour operator Quicksilver Group said lifeguards and an off duty doctor performed CPR but the woman could not be revived.
The woman was wearing a life jacket and had been swimming in front of lifeguards.
"Our thoughts are with those affected by this incident and their families," Quicksilver Group managing director Tony Baker said.
The woman was part of a 25-strong Spanish tour group who were among 80 passengers travelling on Quicksilver Group's luxury 32m Ocean Spirit catamaran for the Michaelmas Cay day trip from Cairns.
"Our staff are trained to respond to medical incidents and we take the safety of our guests very seriously," Mr Baker said in a statement.
"We will assist the authorities in any way we can and work with them to ensure the safety of our region so visitors can continue to enjoy snorkelling activities.
"Counselling will be provided to our staff."
Queensland Police said they did not have many details but were not treating the woman's death as suspicious.
They will prepare a report for the coroner in conjunction with Workplace Health and Safety investigators.
"It may be some time before police can confirm the identity of the woman and notify their next-of-kin," police said in a statement.
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