Here are some possibilities:
Kato Marine in Annapolis can do custom davit, pole support fabrication.
Marine canvas suppliers usually know who can do stainless welding and are used as subcontractors on their projects. Talk to one near you.
The rigging department at Port Annapolis Marina can do what you want, but that will probably be the most expensive.
A canvas shop on your side of the bridge- Rock Hall Canvas Works maybe, would be your best bet on a lead to a SS welding shop.
And I have found that if you start with 316 electropolished SS tubing and grind and polish the welds, they look great and hold up well to corrosion. Not cheap to do right, though.
David