Apparently from Watson's statement to police, he appeared nervous that other people could see him:
"The oddest part of Watson's statement related to the critical minute before he separated from Tina for the last time. He said he was breathing hard and feeling heat on his face, and sensed someone watching him.
"I was thinking those people could see us or at least think something odd was going on," he said. But earlier Detective Senior Constable Gehringer had asked him if there were any other divers nearby.
"I don't think there was anybody'" Watson said."
Source:
Gabe Watson 'hung out and asked for hugs' on other dive boat as others tried to save Tina's life | The Courier-Mail
The odd thing about that statement could be interpreted as Watson fishing to find out if anyone saw anything and what did they see, supposedly so he could form his story to match.
Regarding milking the trip for all the dives before killing someone - I suppose you could say if he wanted to get it over with and that would be a good reason (in Watson's mind) for someone to think he is innocent. Maybe by the end of the trip, Tina would have more experience and be able to defend herself better.