Where are the save files (.rgd) located in the computer? If they can be accessed, can they be copied and transferred onto other devices - such as from Windows to Mac - and continue playing from there?
To find your save files on Windows you have to go to your appdata then go into the LocalLow file then into the Raft file then into the saved games file, you can copy and paste your saved games into this file and your game save should show up when you run the game.
EX: C:\AppData\LocalLow\Raft\Raft\SavedGames