My first thought was a Ludwigia sp. too. As far as positive ID I don't know. I have seen this plant when out looking around a lot. In this picture it is much greener but it was very early spring and in a shaded area. Out on the edges of the shore in full sun it tended to be more red in color.
I would recommend a book if you are a serious Texas ditch diver. Aquatic and Wetland Plants of the Western Gulf Coast by Charles D. Stutzenbaker
. I have it but its packed up at the moment or I would have cracked it open to assist in the ID.
Not much help I know but its all I got at the moment.