We can't sugar-coat this, and it probably qualifies as a disappointment, but if you need supplies while docking in Kingston, from most marinas you'll need to hike or take a cab. There is simply nothing within convenient walking distance for you.
Well, one bright spot. The grocery store in 'downtown' Bath now sells beer & wine. If you stop at Loyalist Cove Marina, its maybe a twenty minute hike. (Hardly convenient, I agree. What's twenty minutes for a beer? Oh! They also have groceries.)
Come to think of it, it's also pretty close from the Flora MacDonald Basin to the LCBO and Food Basics store, isn't it?
Here's a map (still under development) of where to go.
1 - Beer store-1
2 - Beer store-2
3 - Beer store-3
4 - Beer store-4
5 - Beer store-5
6 - General store
7 - Groceries-1
8 - Groceries-2
9 - Liquor store-1
A - Liquor store-2
B - Liquor store-3
C - Liquor store-4
D - WineRack-1
E - WineRack-2