Hey we are not sure how to do it on Mac but on windows you can delete Raft in the windows register at this location
HKEY_USERS/S-1-5-21-2230790418-1291474799-221512870-1000(Or similar)/Software/Raft
If you can find the similar place on Mac and just delete the Raft folder the game will create a new one when you start the game.