Tex. Water Code § 57.217

A district's bonds, when certified and approved by the attorney general and registered by the comptroller as herein provided, shall be legal and authorized investments for all banks, trust companies, building and loan associations, savings and loan associations, insurance companies of all kinds and types, fiduciaries, and trustees and for all interest and sinking funds and other public funds of the State of Texas and all agencies, subdivisions, and instrumentalities of the state, including all counties, cities, towns, villages, school districts, and all other kinds and types of districts, public agencies, and bodies politic. A district's bonds shall be eligible and lawful security for all deposits of public funds of the State of Texas and all agencies, subdivisions, and instrumentalities of the state, including all counties, cities, towns, villages, school districts, and all other kinds and types of districts, public agencies, and bodies politic, to the extent of the market value of the bonds, when accompanied by any unmatured interest coupons attached to them.