Hi, greengage,
I’m not entirely convinced by this clue. The answer appears to refer to a more or less inland sea adjoining the Crimean Peninsula. If you Google “water fern”, you will get a six-letter word ending in “lla”, that is, “all over”. Replace these three letters with V, and you get the sea.
My concern is that, although V is the Roman numeral for five, Chambers doesn’t specifically mention “fifth”. Is there an alternative parsing that makes sense?