'Simon Platts, who directed a BBC documentary episode about the Titan sub last year, said if the vessel was on the water surface it would be really hard to spot.
He said normally the sub comes to the surface and gets in touch with the mother vessel, before they both travel towards each other.
It is a "very small thing in a big ocean" because only the top of the sub floats above the sea level, he said.
The director of the BBC's Travel Show said he couldn't even spot the sub when they were right next to it and somebody had to point it out.'
That would be even worse physiologically for them bobbing on the surface and suffocating than being on the ocean floor.