Mo. Rev. Stat. § 306.260

All marine toilets on any boat, operated upon waters of the state, shall be so constructed and operated as to contain all sewage aboard the boat and not to discharge any sewage into the waters directly or indirectly. No boat shall be equipped to permit discharge from or through any marine toilet, or in any other manner, any sewage at any time into waters of the state, and all sewage when removed from any boat shall immediately be placed in an approved septic tank, sanitary lagoon or sewage treatment system. The provisions of sections 306.250 to 306.290 shall not apply to boats engaged in interstate commerce on the Missouri and Mississippi rivers.