The Kayak Anglers Tournament Series (KATS) will be having an event Sam Rayburn on 3/24 and at Houston County on 5/19. KATS is the oldest and largest tournament series in the country, with a Social and Pro Division. Expect anywhere from 80-120 participants at either or both events. For more details, visit fishkats.com.
There also the Texas Kayak Tournament (TKT) series in the Austin area during spring and during the summer months there's the Thursday Night Tournaments (TNT) series. Duc Tran is running a series for North Texas kayak anglers. Lots of options if you don't mind driving a bit. At some point, Houston will have their own series, I'm sure.