Beth French, 36, from Milverton has set a world first by swimming the 26 miles from Cornwall to the Scilly Isles.
The massage therapist from Milverton, Somerset, encountered seals, dolphins and large jellyfish during the challenge.
Ms French – who trained by pulling her five-year-old son Dylan behind her in a dinghy – set off from near Penzance at 12.20pm on Tuesday.
She finally reached a beach on St Mary’s in the Isles of Scilly at 5.48am on Wednesday, 17hrs and 28 minutes later.
Speaking after her achievement, Ms French said: “I was determined to finish, there was no question of not achieving it.
Ms French said the challenge had been “one of two halves”, with the first 12 hours passing relatively without incident.
“At 12 hours I got a dose of fumes, I ingested too much diesel from the boat,” she said. “That led to spasms, made my knees hit my chest and I couldn’t swim. That was followed by uncontrollable retching. I actually came very close to ending the swim. It was completely pitch black and I started to get cold.”
She added a small number of swimmers have previously made the crossing, starting from the Scilly side, but she is the first to complete it in the opposite direction.
Ms French was observing English Channel rules during the swim – meaning she could not use flippers, a wetsuit or drysuit.
“There was an awful lot of jellyfish,” she added. “They stung regularly but not badly, after a while I managed to ignore them. “There’s not an inch of my body that hasn’t been stung.”
However, Ms French said some sea animals did provide her with some comfort, with dolphins making noises around her. “I could hear them clicking in the sea and that was a great comfort to me – it really buoyed me up,” she said.