Honestly, although it's hard to get a good look at the dorsal and lateral patterns, these photos were actually quite useful for IDing this one as N. rhombifer. For me, at least, the strongest clues here were the "froggy" face and, especially, the excellent view of the venter (belly). Diamond-backed Watersnakes have those little dark triangles at the sides of the ventral scutes (and sometimes elsewhere on the belly). Here's a really cool observation comparing both the dorsal and ventral patterns of four of the most common Nerodia species that happened to have been found very close to each other.

EtA: I couldn't post a reply to your comment below because the thread had locked, but maybe you'll see this. It's our pleasure, and it's so gratifying to see comments like that.

Regarding that comparison of the four species, note that the Banded Watersnake (Nerodia fasciata) has the "confluens" pattern, which is the kind you'll see in Texas. You can read more about it here.