Definitely a rat snake. Totally harmless and actually quite beneficial. I always let those guys go on their merry way. Also called a chicken snake by some but there are several species that are commonly referred to as either rat or chicken snake. Water snakes, those found in Texas, tend to be a lot shorter and fatter. I can't think of any water snakes at all with those markings.
Saw my first actual water moccasin recently. I grew up "up North" (sorry) and every water snake I saw I was told it's a water moccasin! Seeing my first one was pretty impressive. Unique kind of snake. I'm glad it was a good 20 feet away though. I also found a Texas Blind snake in our pool and released him. That was interesting.
From that far away, it could be anything. I understand most people don't want to get close enough to take a good picture though. Rat snake was a good guess. Could also be diamond back water snake. Google the pics and maybe you'll recognize something. It does not fit the profile of any native Texas poisonous snake.
