Continuity mistake: When Dickie and Tom go out on the boat, Dickie is wearing his white jacket. When Tom swims back to shore, the white jacket is dry on the rocks as if Dickie hadn't taken it with him.
Continuity mistake: When Tom is with Peter on the boat sailing to Greece and Tom says he's going to watch the sunset, the sun is just above the horizon. When Tom goes into Peter's room after talking with Marge, you can see the sun in the exact same position out the porthole window in Peter's room.
Continuity mistake: When Tom is taking Freddy's body out of his building, Freddy's coat is buttoned to the top. When Tom puts Freddy in the car, his coat is suddenly undone and blood is visible on his shirt. (01:28:00)
Continuity mistake: When Tom is playing the piano in the apartment, he has both of Dickie's rings on, but when he gets to the door when Freddie is there, he only has the pinky ring on the left hand. The gold middle finger ring is now missing when ne puts his right hand up on the door, but he never took it off. (01:21:00)
Continuity mistake: Tom has a scrape on his face as he is typing the "suicide" note in the apartment. But as he is leaving and peering around the corner of the building, the scrape is gone. (01:38:00 - 01:44:00)
Continuity mistake: From the viewpoint of a boat behind Dickie's boat "The Bird", when Dickie gives the tiller to Freddie, so he can go below with Marge, there's hardly any wind, both sails are flapping around, but when we get the view from midships, the front sail, which was on the starboard side, is now, suddenly, on the port side and full of wind, no flapping around at all.
Other mistake: Tom Ripley's ticket says first class in the limo, but when he boards, the rich girl says she saw his stuff in the third class stack. Dickie also says that he's a third class mooch, but according to his ticket he rode first class, so that'd make him a first class mooch, not third. His cabin is never shown. (00:07:20)
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.