From the looks, polders will likely need 2 adjacent land tiles, but my guess is they remain a "sea" tile for everything else. Again, interesting to see whether they will gain harbor bonuses. Otherwise, I like a lot about that civ. Extra adjacencies are nice, and polders definitely look like pretty sweet tiles, meaning that you definitely want to settle on or near coasts. I don't think they'll necessarily come out as a top civ, assuming that polders come late enough in the tech tree to not be super-OP early improvements. But they look like they should be a fun one to play.