Ace in Cole Bay sells chairs and umbrellas, yes. Are you planning on staying only on SXM? As I don't know about other islands, but technically all beaches on SXM are public beaches, so they can't prevent you from bringing your own chair. But you can't set your chair up in front of chairs that are already there. It really depends on what beaches you intend on going on. There is always a ton of room at Mullet, for example, and there are some relatively deserted beaches like Happy Bay that you might be able to get to from your boat, with a dinghy, and bring your own chairs and umbrella. Club Orient beach, you can also bring your own chairs and umbrella, but again, set up behind the other chairs. I don't honestly know, as I don't do sailing, but the charter boat doesn't have any chairs and umbrellas onboard?

With regard to provisioning, I'm not sure what the exact question is?