Yet my wildflower guide is telling me it isn't in that actual genus.
It is, however, what is commonly called a mallow (as are some hibiscus). And it is commonly found at the seashore. Ergo: Seashore Mallow. The lovely little plant (the flowers are barely two inches across, and on stems about two feet tall) is native to the region (hooray!) and is spreading beautifully all by its ownsome over our little bit of saltmarsh in Avalon.
Isn't it amazing what shows up after you stop applying herbicide to keep "weeds" out of a stone yard?