Searchers struggled to remain positive as the search for missing Toronto filmmaker Rob Stewart off the Florida coast continued Thursday.

About 20 boats and 12 aircraft operated by volunteers have joined the search, which also includes members of the U.S. Navy, the Florida Fish and Wildlife Conservation Commission and the Monroe County Sheriff’s Office.

“The search conditions continue to be great,” Stewart’s sister, Alexandra, told the Star on Thursday afternoon.

A Coast Guard team remained on the water overnight, looking for traces of the 37-year-old as the search entered its third day. Stewart hasn’t been seen since 5 p.m. Tuesday, when he didn’t resurface while scuba diving in an area called Alligator Reef.

Stewart’s sister said that no evidence has been found of her brother.

“We’re feeling terrible,” she said. “The search on the surface continues strong.”

Three helicopters were part of the search on Thursday, Coast Guard spokesman Nyx Congemi said.

Coast Guard officials said there’s no set time when a search would be called off.

“As long as there’s a chance for survival, we’re going to search,” spokesman Jonathan Lally told the Star Thursday afternoon.

“We remain hopeful we’re going to find him alive.”

Helicopter crews resumed their efforts as the sun rose Thursday, the Coast Guard said.

The type of diving suit Stewart was wearing makes it difficult to search for him with thermal imaging, so that type of search was put on hold overnight, according to an update posted to Stewart’s Facebook page by his friends and family.

Stewart, best known for his documentary Sharkwater, was in the sunshine state to film a sequel called Sharkwater: Extinction. Friends say he was diving near the Queen of Nassau shipwreck in the Florida Keys and was less than 10 kilometres from shore when he apparently slipped underwater.

“He had surfaced and gave the OK sign and then he was gone,” his sister, Alexandra Stewart, said Wednesday.

“The other diver got on board and was struggling to get on board and then fell unconscious.”

The crew of the boat lost sight of Stewart amid the commotion, she added, and there was concern her brother had lost consciousness as well.

The dive was her brother’s third of the day, she said, noting that he and his diving partner had gone down about 70 metres.

Stewart is an experienced diver who started underwater photography at age 13 and had qualified as a scuba instructor trainer by 18.

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Stewart’s sister and parents travelled to Florida when they heard of his disappearance, and are now helping manage search efforts.

A gofundme.com page has also been set up in case search efforts are prolonged. By early Thursday morning, the site had collected more than $90,000 in donations.

With files from the Canadian Press