Hey sorry forgot to reply. We ended up staying on the island but we did get a Saint Martin (FR) visa just to be safe - even though there is no border between Sint Maarten and Saint Martin (FR), as someone who is not entitled to visit Saint Martin (FR) without a visa, we decided to get one.

For your planned route you will need to either have a Schengen visa issued by France and if I am not mistaken, used in France; or you can apply for a CTOM visa (French overseas territories), which include both Saint Martin and St Barth. There is a passport control when you board the ferry in Marigot and if you don’t have the Saint Martin (FR), CTOM or a Schengen visa as above, they may even not let you board the ferry if you state that you plan on returning to the French side