H. W. Smyth, Greek Grammar para. 1323, calls it the "genitive of material or contents," and cites Xenophon with κρήνη ἡδέος ὕδατος a spring of sweet water and σωροὶ σίτου, ξύλων, λίθων heaps of grain, wood, stones. Here, putting κακῶν before the noun is just a matter of emphasis; it doesn't change the meaning, just puts emphasis on "evils" rather than "sea."