The Polder turns Marshes into 4 food tiles, and flood plains into 5 food tiles once you research Guilds. this is actually better then any farm on a non-river tile (marshes) and a civil service farm on a flood plain. The later boost that comes from Economics essentially gives the polder the effects of a trade post with an extra production. In the end the Polder is a very high yield tile improvement that becomes available in the medieval era, and matures in the renaissance. It essentially turns a rather useless tile (marshes) into a very powerful one that will provide whatever city works it with a really well timed mid-game boost to city growth, just as happiness buildings/resources come available and allow you to grow a wide empire past 6-8 pop
That's exactly right. The problem with my Dutch empire is the fact that I can only build polders near three cities. One of those on only one tile. That is probably a good thing to make sure it is not overpowered. Of course, I could have just had bad luck with map generation.