They can't do it, Jim...
Federal law specifically prohibits any state from enacting a law that goes beyond federal law:TITLE 33,CHAPTER 26, SUBCHAPTER III, section 1322"Marine sanitation devices(f) Regulation by States or political subdivisions thereof; complete prohibition upon discharge of sewage(1)(A) Except as provided in subparagraph (B), after the effective date of the initial standards and regulations promulgated under this section, no State or political subdivision thereof shall adopt or enforce any statute or regulation of such State or political subdivision with respect to the design, manufacture, or installation or use of any marine sanitation device on any vessel subject to the provisions of this section.(B) A State may adopt and enforce a statute or regulation with respect to the design, manufacture, or installation or use of any marine sanitation device on a houseboat, if such statute or regulation is more stringent than the standards and regulations promulgated under this section. For purposes of this paragraph, the term “houseboat” means a vessel which, for a period of time determined by the State in which the vessel is located, is used primarily as a residence and is not used primarily as a means of transportation."So before TX can require gray water holding from all vessels, federal law would have to be changed.However, the TX regulations concerning "sewage" ("human body wastes and the waste from toilets and other devices intended to receive, retain or discharge human body wastes") are well within federal guidelines...in fact, in some states--GA is one--it's not enough to just disconnect and plug any lines connected to macerators, y-valves and/or through-hulls...they must be removed and the seacocks permanently secured.