Danielle Hooker, 28, fought in vain to keep her unconscious boyfriend Rav Pillay afloat and alive with the assistance of another survivor as they awaited rescue after the Leviathan II sank near the resort of Tofino, Vancouver Island.
But Canadian police have said how in the heavy swell, and growing exhausted, Hooker was unable to save both herself and Pillay.
"It's him or them," said police spokesman Constable Jim Preston.
Hooker also lost her father, Nigel Hooker, 63, from Southampton, while her sister Aimee survived the capsizing of the vessel after local fishmen picked them up.
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Pillay's body was still missing today after police divers suspended the search because of poor weather. Five others who drowned were British. A total of 21 people were rescued from the Sunday mishap.
Pillay, who lived in Sydney, was born in South Africa and grew up in Durban. His parents, Mike and Judy Pillay; his brother Kereshan; and his sisters Surina and Survana updated their social media profiles to feature pictures of Pillay.
Nigel Hooker's family issued a statement saying they were in "complete shock and disbelief and struggling to come to terms" with his death.
The investigation into the disaster is focussing on why the vessel proved so unstable that it capsized when hit by a wave.
Canada's transportation safety board said the boat turned over while most of the passengers were gathered on the upper deck watching sea lions, which made it top-heavy. But the owner of the boat, Jamie Bray, said the vessel had made the same trip every day for 20 years without incident and the skipper had 18 years' experience.