Here is an interesting beach guide put out by the Texas General Land Office believe it or not. It’s a nice guide because a lot of the lesser known beaches, parks, boat ramps, etc. are shown. Generally you’ll have to Google to get more info, but this is a great place to start especially if you’re looking for an out-of-the-way place.
Go to the COAST tab in the index and you’ll find a host of resources centered around Texas coastal waters. On in particular that might be of interest is the Texas Beach Watch page where they report on water cleanliness.