1.5 hours is plenty of time to get to the ferry dock from SXM airport, regardless of which dock you are departing. I highly recommend scheduling a taxi ahead of your arrival, so that your driver will instruct you on where to meet him/her. They will get you straight to the dock and most likely provide some beverages along the way. Once at the dock, be prepared to show your passport.
The only possible delay you may have is a wait for luggage on arrival. SXM airport can be a bit slow.
Enjoy!