July 2012. When a Pestnet member first saw the plant he thought it was Vigna marina, but the flowers of that are yellow not purple. This plant grows on sandy beaches, and can overgrow and kill coconuts. When it was first seen it was thought to be Vigna marina, but the flowers on that are yellow, not purple.

A member in St Lucia wrote that the plant was Canavalia rosea(Swartz) de Candolle, Prodr. 2: 404. 1825. Bas.: Dolichos roseus Swartz (1788). Syn.:Canavalia maritima(Aublet) Urban (1919). It is a member of the Papilionaceae.

C. ensiformis and C. speciosa were also suggested, as well as C. cathartica, see:

http://www.hear.org/starr/images/species/?q=canavalia+cathartica&o=plantsand http://www.flickr.com/photos/jbfriday/6473416083/in/set-72157625624874013
It is naturalized in Hawaii but not native.