Well, I got certified to dive warm, clear water over coral reefs, so I don't get too enthusiastic about Texas freshwater diving. Doesn't hold a candle to the diving I have done in the BVIs, Martinique, Zanzibar, etc., but here is the sum of my Texas diving experience:
Windy Point, Lake Travis - got my NAUI Open Water cert there in Nov. 1995 as a sophomore in college, and then was there again for my Advanced in 2002. 'S okay, I guess.
Twin Lakes, Manvel - got my Rescue there in 2008. Wasn't terrible, but not a place I would go to if I wasn't taking a class there.
That's pretty much my feeling about Texas freshwater, (and I have also been to (though not dived) Canyon and Possum Kingdom), these just aren't places that are worth me driving to to go diving in. And I have no interest diving the Blue Hole in Huntsville. I don't care how clear the water is, I dive to see fish. The one exception would be Balmorhea. I went there this summer for the first time, and had a great experience. In fact, even though I have been diving in premiere dive spots all around the world, I would list it as one of my top 10 diving experiences. Here is the review I have written of it for other websites:
Just got back yesterday from a 9 day trip to the Trans-Pecos region of far West Texas. A highlight for me was scuba diving in Balmorhea State Park with two critically endangered fish species - the Pecos gambusia and the Comanche Springs pupfish.
It really was amazing, starting out from my home in lush, green, forested, humid Houston, quickly crossing into blackland prairie and post oak savannah in the counties just west of the Houston area, then moving up to the juniper-and-oak-dotted limestone hills of the Edwards plateau, before finally entering the mountain-basin region of cholla cactus and mesquite that is the Chihuahuan Desert of far west Trans-Pecos Texas. But even out there, the landscape surprises, with contrasts between plains, mountains, and Rio Grande river bank. Though the temperature down on the plains reached 106 F during my trip, at the same time my ponderosa pine-shaded campsite at Chisos Basin in Big Bend National Park never got above 86 F in the heat of the day, and dropped almost 20 degrees overnight. Similarly, during my stay at the CCC-built Indian Lodge in Fort Davis, temperatures never got much above the mid-80s during the day.
It was during my stay in Fort Davis that I visited another CCC-built relic from the 1930s, a true oasis in the desert, Balmorhea State Park. The park is home to the world's largest spring-fed swimming pool. The pool covers 1.75 acres and holds over 3.5 million gallons. It consists of two "arms and a central "bowl". One "arm" and a portion of the "bowl" are concreted in like a conventional swimming pool, and are shallower, ranging from 3-8' in depth, I'd say. The second "arm" which contains the diving boards, and the rest of the "bowl" are not concreted in, and thus have sand and water plants covering their bottoms, and are about 25' deep.
22-28 million gallons of water from the San Solomon springs feed into the pool daily, maintaining a narrow temperature range throughout the year. On the day I went, the water temperature was 77 degrees, warm enough that I didn't need a wet suit (and I am a wimp when it comes to cold water). I didn't notice any temperature differences between the surface and the bottom, probably from mixing due to the spring feed. The water is crystal clear, a beautiful blue color, with the best visibility I have ever seen scuba diving, and I have been diving all over the Caribbean and in the Indian Ocean as well. I say without exaggeration it truly was swimming pool clarity.
The dominant fish in the water were Mexican tetra (Astyanax mexicanus). These gregarious fish seemed as curious of me as I was of them, and would swim along right under me as I swam the length of the pool. They would also nibble at bubbles on my arm hair. They had fairly bright iridescent stripes along the sides of their body, and most were 3-5" in length. Next in quantity were the two endangered species, the Pecos gambusia (Gambusia nobilis), and the Comanche Springs pupfish (Cyprinodon elegans). These two fish were so plentiful in the pool it was hard to imagine that they could be endangered, until I remembered that the pupfish is found only in the Balmorhea, Texas area and nowhere else.
The western mosquitofish (Gambusia affinis), is by far the most common fish I catch in my freshwater collecting around Houston. They do quite well in my garden pond along with the golden topminnows and sailfin mollies I am usually targeting when I catch them, and with the various smaller sunfish species that also inhabit my pond. However, I have never thought of them as very pretty fish. "Basically drab guppies" is the way I describe them to non-fishy people who ask me what kind of fish they are. That is why I was really surprised at how pretty the Pecos gambusia is - a bright orangy-yellow that seems to glow because of their semi-transluscent bodies.
When I collect in the saltwater marshes of Galveston Bay, I commonly collect the Comanche Springs pupfish's cousin, the sheepshead minnow (Cyprinodon variegatus), and I have always found the males in breeding season to be quite attractive - olive elongated blotches vertical on the body, an iridescent blue head, and rosy-orangish pectoral, anal, and caudal fins. The pictures I have seen of desert pupfishes tended to look more drab than this, with very little in the way of markings on the body, but sometimes with iridescent blue that extends on the entire body. I was really surprised, therefore, to see that the Comanche Springs pupfish, while identical in body shape to the sheepshead minnow, actually has very interesting markings, very similar to the markings of lake trout (Salvelinus namaycush). The gambusia and pupfish were plentiful and highly visible, but seemed less interested in me than the tetras, and mostly ignored me as they poked through the lush underwater vegetation for food.
Other species I observed were crayfish and some very dark catfish that were usually only about 1-2' in length. It was also very cool to loiter over the springs themselves, watching them bubble up through the sand. The tetra seemed to like this too, as there were always a lot of them playing in the springs. In all, I spent probably a little over an hour of submerged time, until my tank was down to 700 psi. With the depth no more than 25 feet and such a controlled environment, it was the most carefree diving I have ever done. If you are ever planning to take the trek out to Big Bend National Park, I highly recommend building some time in your trip to head north to visit Balmorhea State Park, just 30 minutes from Fort Davis (Fort Davis itself is a worthwhile destination in the beautiful Davis Mountains). It is best experienced by scuba diving, but I think snorkelers would enjoy it too. Even if you can't swim, the wetlands just beyond the pool have a viewing window where you can see pupfish.
Diving is free with your admission to the park. I rented my BC, octopus, tank, and weight belt (brought my own fins and mask) from for $50 from Darrell at the Funky Li'l Dive Shop just across the street from the park. And if you are travelling without a certified dive buddy, don't worry. I was travelling with my non-diver wife and 6 year old daughter, so I had asked Darrell if he could hook me up with a dive buddy, since the park website said "All individual scuba diving will be done on the buddy system. A minimum of two (2) certified divers is required to dive." Darrell said not to worry, even though the site says that, as long as you have someone at the pool with you watching out for you, even if they are just sitting on the side, you are covered, and he was right. When I checked in and signed the diver release form, I was not asked about a dive buddy.
I dove in just in my swimsuit and felt fine all the way to 30'. I have heard they are going to be closing North Park at the end of the month though for the season. Upon talking to a local diver they said that Turkey Cove (or something to that effect) on the opposite side of the lake stays open year round. 
