This is the beautiful flower of a common beachside bush (Thespesia populnea) called the haiti-haiti tree here on St. Croix, also known as seaside mahoe, portia tree, and often mistakenly called beach hibiscus. *
Despite the common name sometimes used, the beach hibiscus is actually a different plant (Hibiscus tiliaceus). Still, the flower does look hibiscus-like. I was attracted by the filmy, creamy translucence of the off-white petals when the flower is fully opened.
While the short-lived flowers are beautiful, the tree itself is scrubby with multiple woody stems. Invasive and salt-tolerant, it can quickly dominate a shoreline. The fruits or seedpods are a favorite of the local bright red love bugs, one of which is featured in my gravatar. More on those guys later!
*Thanks to Carol Cramer-Burke at the St. Croix Environmental Association for pointing me in the right direction on the facts here.