Just returned from a sailing trip with DYC SXM 2 weeks ago - sigh.
DYC offers a provisioning service that I used and was happy. I ordered all the large cumbersome items I didn't want to schlep to the boat such as water, sodas, canned goods etc.
For the items I wanted to purchase in person I walked to the SuperU in Marigot - about 15 min. Was able to purchase everything I required.
In addition Serefinas is an amazing French bakery in Marigot. Def go there. I ordered custom sandwiches to eat underway amoung many other wonderful pastries!
Enjoy, it's a fabulous sailing area!
Ps: I'm replying from my phone, please forgive my errors. I don't have access to a computer as I'm not home.