'Kastra' is the plural form of 'kastor'. For adjectives in the aquatic (and terrestrial) gender, the form is the same in both nominative and accusative (singular or plural) :)
So, for instance, if we switched 'blue ocean' and girl around we'd then have 'Riña kastor embri ojehiksa'.
I was under the impression that 'kastor' was used to describe accusative objects. I don't get when 'Kastra' gets used.