See here is the thing, they would have been at the surface when stopped. They would also have had explosives on board, the cover story was lnt going to hold up under a lot of scrutiny, it was meant for the coastguard.

They could have had labelled the mixed gases as standard tanks or shown the additional equipment as part of the dive centre boat. Coastguards also dont have to check dive certifications, it is not illegal to dive without them.

If I was hiring for an underwater porn I would hire people with a good level of skill and comfort in open water who want to show their bits.