This can be looted or bought in: Inkopolis square and plaza. Some can be found in underwater dungeon chests.

Splat Charger

Firearm "This weapon can only be used by Inklings and Octolings. This weapon is a martial ranged weapon." Common [None]

A brightly colored sniper-rifle water gun of sorts, which shoots out the PC's color of ink. It must charge up before firing, & It takes one turn to charge it fully, and it will shoot a line of ink, leaving a trail along the ground. It can hold a charged shot for an unlimited amount of time above ground, but if the PC wishes to swim in ink while holding a charge, the held charge will disappear within a turn. It’s ammunition comes from the ink tank held on the backs of the PC. The PC must wait in the ink an x amount of rounds in ink to refill their tank, x being decided by the DM and dependent on how much of the tank they want to fill up. The splat charger can shoot certain surfaces, and then the PC can swim through the ink on the ground. Here is a table of surfaces the  splat charger can ink.

Cost: 4gp Weight: 6lb