We conventionally call the Earth's seas "water" despite the fact that they have a lot of dissolved CO2 and fish in them. Presumably Ceres's seas don't have any

We conventionally call the Earth's seas "water" despite the fact that they have a lot of dissolved CO2 and fish in them. Presumably Ceres's seas don't have anything like a fish but without microorganisms to scavenge they'll tend to have a lot of carbon containing compounds.