The Saltwater Gun is a single-use weapon in Mother 3. It is the equivalent of the Rust Promoter in EarthBound and works much like a water gun, being super effective against mechanical enemies such as the Nice Poser and Whatever.
The Saltwater Gun can be obtained as early as Chapter 4 from the mouse in the Club Titiboo Attic. It is also sold by a Pigmask arms dealer on the Highway and in New Pork City. It also works against slugs.