Abstract: A unit for injecting a disinfectant, deodorant or the like into the inlet stream of flushing water as it is pumped into the bowl of a tankless marine toilet. The injection unit is preferably constructed of commercially-available plastic plumbing components. The main body of the unit provides an additive reservoir which surrounds the flush water inlet pipe and is formed from a conventional plastic sanitary tee jointed fitted at each end with plastic size-reduction bushings. The two bushings connect the injection unit to the flush water inlet tubing and also support an interior flow pipe. Holes drilled at angles in the flow pipe divert a portion of the flush water through the additive reservoir where it mixes with and dissolves the additive (normally a water soluble disk of the type typically placed in the tank of a household toilet).