A woman who could not swim was forced to cling to the hull of a capsized boat for hours while her companion swam through rough waters near Hervey Bay to raise the alarm.

The pair, aged in their fifties, was on board a small catamaran that overturned off the coast of Point Vernon on Sunday afternoon.

Emergency services received the distress call about an hour-and-a-half later after a man swam up to a kilometre to shore.

Vice commodore Jill Barclay from the Volunteer Marine Rescue said the boatie had done well to make it through rough seas.

The man swam up to a kilometre back to shore through rough seas. ( Supplied: Marine Rescue Hervey Bay )

"It could have been anywhere between 500 metres and a kilometre back to shore, so quite a substantial swim for the person to get back to shore to raise the alarm," she said.

"The vessel was probably drifting with the wind and the tide — sea conditions were actually quite rough at the time out there as well."

Ms Barclay said a rescue crew was dispatched and the 57-year-old woman, Tasha Irvine, was found clinging to the boat a short time later.

"As you'd expect after being out there for that length of time, [she was] in a bit of a distressed state," she said.

Volunteer Marine Rescue described the operation as "a challenging mission". ( Supplied: Marine Rescue Hervey Bay )

"We did call an ambulance and took her to hospital as well just to get her checked out."

Hervey Bay Water Police officer-in-charge Paul Bacon said there had been serious concerns for the woman's safety.

He said the vessel could quite likely have sunk in the time it took rescue services to arrive at the scene.

"I think the whole situation having someone left on the boat, having to swim for help, would be quite a significant shock to the system," he said.

"They've done extremely well to make it into shore but I suppose to a certain extent, planning with that, having safety equipment on board, life jackets, certainly aided in the successful outcome."

Ms Irvine took to Facebook to thank the rescue crew for coming to her aid.

"I am very proud and very, very grateful to all involved — I am alive today because of your awesome efforts," she wrote.

"Suffice it to say my boating days are over, even as a passenger, but again thank you all so very, very much."

The rescue boat returned to the catamaran and towed it back to the Gatakers boat ramp.

In a Facebook post, the Volunteer Marine Rescue described the operation as "a challenging mission … given the weather and circumstances".