Cradily is a fossil Pokemon, and all fossil Pokemon are park rock type.
Cradily might live in the sea, but that doesn't mean it has to be water type. It's called the Barnacle Pokemon for a reason; it's supposed to resemble real life sea anemones. Cradily is also revived from its fossil, which means that it's a Pokemon that lived thousands of years ago.
Anorith is exactly the same. It's Bug/ Rock but lives in the sea and even has Swift Swim as its hidden ability, but isn't part water because of the same reasons (it's a fossil Pokemon).