Unless Tom threw the body overboard and the Italian check-in list was sloppily prepared, there's no way Tom wouldn't be linked to Peter's death. Even his disappearance would be suspicious since Marge and her father knew they were going on Peter's tour together. End of the road, Tom.
shareYes, Tom would have thrown the body overboard, and yes, the Italian documentation would have been as haphazard then as now, but Peter was expected to perform in Athens, and yes, Tom had stated that he was joining Peter; also Meredith had seen him on the boat.
Tom's only hope of escape would be to disappear once they docked in Athens until the trail was cold. But Tom would have been poison in the Anglophone expat community, because Marge would have broadcast her suspicions to everyone around her.