Well, a lot of Alternian biology is total guesswork! But to cut a lot of complicated sciencing short, saltwater fish on Earth are configured to always be getting rid of excess salt due to being surrounded by the stuff constantly, and have no way to get rid of excess water other than by osmosis, which is when less salty water wants to absorb into where more salty water is, a process I still don’t get despite watching like a dozen graphs and videos. But anyway they’re always losing water actually, and so they cope by drinking a lot of it and just filtering out the salt. But when the saltwater fish are dumped into fresh water suddenly they’re more salty than the surrounding water, and so the surrounding water sucks into them, and they have no way to purge the water, so their cells bloat up and rupture.
So I figure, it’s likely that a seatroll would get thirsty out of the ocean, but do best drinking saltwater that their body can then strip of whatever level of salt is best. Drinking freshwater wouldn’t fuck them up directly, but neglecting their sodium levels—which would probably fall faster than a landtroll’s—would. And breathing freshwater would probably result in the delicate, absorbent tissue there puffing up from retained water in the way that the tougher tissue of an esophagus wouldn’t. I think over some period time the extra water in his gills will diffuse out to the rest of his body tho. The extra mucous is just a regular reaction of sensitive breathing parts to being exposed to a harsh chemical. You try huffing chlorine and see what happens!
So… there you go. Roaming seadwellers need to consume a lot of salt, but can drink plain water fine in moderation, but breathing it for too long is asking for some gross bloating, and breathing chlorine or pollutants is gonna give you a case of the sniffles.