Visible crew/equipment: When Dickie and Ripley are scootering down a little Italian hillside, you can see the shadow of the whole film crew against the hill.
Visible crew/equipment: When Dickie, Tom and Marge are coming out of the Venice Railway Station via revolving glass doors, the cameraman is visible in its reflection on their left when it closes.
Answer: After running into Meredith, Tom's plan was to stay in the cabin with Peter the entire trip to avoid them seeing one another. But after Tom makes that suggestion, Peter tells Tom that he seen him kissing Meredith. He couldn't avoid or kill her since Peter already saw her. Family members also saw Tom with Meredith. If Meredith suddenly vanished or was killed I'm quite sure there'd be an investigation. Peter was the only option. No one is on board to report Peter missing. Once the boat docks, Tom will be long gone before they discover Peters body. Unlike Meredith, her family would be looking for her immediately since they're traveling together. I'm quite sure he'd much rather have killed Meredith, she meant nothing to him.