Ludwigia peruviana
aka Peruvian Primrose-Willow (id assistance courtesy of Bill Diffin)

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Ok, I think the heart-shaped leaf in the background may belong to the white morning glory in the previous image. The vine is obviously sneaking through various plants and the flowers show off while the leaves hide. Meanwhile, I don't know what this yellow flower is.

Looking up primrose florida (bd suggestion) - primrose seems to give both ludwigia and oenothera genera.

See:
http://www.floridasnature.com/florida_herbs_11.htm
Largeflower Primrose-Willow, Ludwigia grandiflora
Family - Onagraceae
Habitat - Marshes, Swamps, Lake & Pond margins
... except for the large flowers... the leaves look the same, and the habitat is right.