There is a ferry that connects Saint Marie to Guadeloupe.
In 1:5 Spot the Difference, a convicted fraudster is stapped to death while handcuffed to Detective Inspector Richard Poole on their way back from Guadeloupe to Saint Marie.
In 4:4 Until Death Do You Part, Detective Sergeant Camille Bordey takes the ferry to Guadeloupe when leaving the Saint Marie Police Force heading for Paris.