In a search and rescue every single millisecond is critical and nobody knows that better perhaps than Survivorman Les Stroud.

His friend, filmmaker and conservationist Robert Stewart, has been missing in the waters off the southern tip of Florida since Tuesday evening but Stroud says he has reason to hope Stewart can be rescued.

"You're breathing moment to moment," said Stroud, who's been following the search from his home in Huntsville.

"Your heart is living on hope. Your mind is battling facts. And you're trying to process it all, and make sense of those two worlds."

Stroud, who's an expert diver himself, says the odds of Stewart's survival, if he was swept away by a strong breeze or current, are good as long as his life jacket is inflated.

'Survivorman' Les Stroud discusses his theory on what may have happened to Toronto filmmaker Rob Stewart who went missing after a dive in Florida. 0:36

"Will he be cold? Yes. Will he be hypothermic? Yes. But will he expire because of those reasons? Highly unlikely," Stroud said.

"The hope is his life jacket, or [buoyancy control device] is inflated, and he's simply out of sight."

Stroud says he's a 'very big fan' of Stewart's work. (Laura Bombier)

Stroud was supposed to be in Florida diving with Stewart, who was filming Sharkwater: Extinction, a follow-up documentary to Sharkwater — his hugely popular 2006 film, which put the spotlight on global shark hunting and its impact on the marine ecosystem.

Stroud, a survival expert who documents his experiences on his long-running reality TV series Survivorman, says the two became friends years ago, when he reached out to Stewart after seeing his film.

"We connected because I have the same passion and same love for the natural world; the same burning desire to save sharks, " Stroud told CBC Toronto.

Stewart is filming Sharkwater: Extinction, the sequel to his 2006 award-winning documentary, Sharkwater. (Instagram)

Stroud: Stewart should be fine, if he's 'floating around'

Stroud says while speculation about what happened is dangerous, he says there's reason to be hopeful Stewart is alive.

Stewart's diving partner surfaced first, after their third and final dive of the day, and struggled to get on board. Stewart, in the water and just behind his partner, gave the crew a thumb's up, a diver's signal they are okay.

Stewart documents his global travel and conservation work on his Instagram account. (Instagram)

"The potential here, with speculation, is that perhaps through the re-breathing and very deep diving, he was as we would say, a bit loopy. His brain was not processing bang on," said Stroud.

"His thumbs up may have been while he was also a bit wonky."