When I was much younger and studying freshwater ecology (late '90's), I went down to 'ol Coleto Creek and had a look-see. There were several areas I noted in late spring that might have hosted decent diving opportunities. Patches of rather clear water around submerged vegetation beds (I forget the kinds) were interspersed near the main boat ramp and out by the dam. The water samples our team took showed very clean, fairly clear water out by the dam especially.
I looked into diving there years ago, but came up empty. It's hard enough finding people who've even HEARD of the place. It really is off the beaten path, unless you're driving to Corpus.
Sailor, why don't you drag your scuba gear with you and check the place out for us? Can't be any worse than some of the other places we often dive.