inspire by xenomorph acid blood from alien movies.
so the water either rivers, lakes, and oceans in my world or some of the regions is highly acidic because some of the dirt or stone minerals content mix with the water make it into corrosive acid that can corrode or melt normal skin tissue upon contact, and during prolong contact it can melt the normal flesh and damage or corrode metals especially iron/steel, and it also can rapidly damage normal wood especially wooden plank not native from this environment, but its not in the point to boil the water or increase/decrease the water temperature by itself nor creating dangerous gas, and rain water is safe though not acidic or not that high or corrosive.
the water also have color (not decide the color yet or will ends up add it in or not later) because some of the dirt or stone and water plant contain some natural dye minerals but its pretty safe for the creature, also the soil outside of the water is in permafrost state (not decide is it mix with the same acid or just normal ice yet, but eitherway even if it contain acid it quite small and not in liquid state so i dont think it will be dangerous or harmful), i add this in case its considered an important information to make better/clearer answer.
so far i assume the water creature can protect their skin or flesh by covering themselves with carapace and other type of shell like crustaceans and Bothriolepis fish, but i dont know how to protect their gills and eyes.
another is base of human stomach using mucus, but i dont know is it effective outside the body or for external organs (at least i think it can protect some of the internal organs or the flesh parts inside of the mouth) but i assumed the mucus can also block the gills making the creature unable to breath or can even still make the creature can see at all because of the mucus covering the eyes.
the aquatic creature not necessary need eyes but i add it just to see the visibility to protect that organs.
also the aquatic creature is form from natural evolution, not from genetic modification by inteligent lifeforms.
and so i want to know what water creature like fish and crustaceans like crab need in order to survive in this environment especially to their gills and eyes, and would there be need a dfferent biological solution or evolution between plain water and salt water.
also curious to see a better solution outside of exoskeleton or shell type ( not exclude it for answer just want to see is there a better possible solution outside of that.)