Yes, get a 1 way, fully refundable ticket from SXM to anywhere. Better safe than sorry. I flew to SXM on a one-way several years ago. SXM immigration asked to see my return ticket. Since I didn't have one, (I was staying several months and not sure of my return date). I was told I would have to return to the US and not be allowed to enter SXM. I was able to talk to a Supervisor, who eventually let me purchase a one-way ticket that satisfied the Agent.