Set in the medieval period around 1400AD, the atlantean set flipper on the land of the giant slug and destroyed many acres of crops, this provocative action angered the giant slugs. These giant slugs on average standing at over 90 cm tall weights half a kilo are hell bent on exacting their revenge had a fear of the sea, how can they protect themselves against saltwater without compromising their already slow speed? They can squirt boiling corrosive liquid at a good distance away about 2 meters, they moved at 2 cm every minute.
It doesn't matter what they wear. They move too slowly to attack successfully and their spit would be completely ineffective underwater. Short of them building attack submarines, they can't win.
That said, a slug would cover itself in slime to protect itself from salt water. If you see salt dropped on a slug, it will generate slime to wash it off. You could have a special slime that is salt resistant.
If the slugs actually wanted to win, they have traps and ambush their enemies when they attacked.
What armor for my giant slug species to do battle at sea?
Your slugs are physically inferior to intelligent vertebrate species. Their tech has had to compensate. The slugs are not about to get into a boat on a sea of deadly salt and battle fast moving aquatic creatures that can easily poke holes in the boat or tip it over. That would be suicide.
Your slugs are going to be protected from the salt, and from their enemies, by staying well above them. Slug airships take a generation to build, but once built are good for many generations more, and they require little movement from the slugs to control.
Your slugs will park above the submerged lands of the Atlanteans. Spraying caustic poison onto enemies makes a lot of sense to the slugs, in a way that "proportional response" does not. Once in position in the sky they will release their toxins. These are not just the biological slug toxins, although they usually add some of that for sentimental reasons. Slug chemists have come a long way as regards poisons, and they have many types. Atlanteans coming to the surface to do battle will wish they had not, long before they reach the surface. Speedy retreat will save individuals, but not their homelands which will have been poisoned in a way difficult or impossible to remedy.