Why ? That theme seems to fit better in
Maybe should be useful with a lowest atk/hp, so you can decrease cost to a midrange attacker with a nice ability ( cost 5 for upgraded and less for the unup)
My reason for this was that Darkness didn't seem to really fit with it, Longship and Dragonships are what vikings used to travel doing their bidding and what not, I figured the
mark resembled it more closely theme-wise. That's just how I see it though, I can always change it to be more fitting. As for cost I'm not quite sure what you mean by that; lower the attack 1 and the cost by 1 as well?
And to Nepycros, I could see that being insanely advantageous, it might even have flooding become more widely utilized.