I have settlers of catan and the water parts of it are bent.The other tiles might also be bent a little bit, but aren't noticeable as much as the water tiles.
How can I make it straight again?
A steam iron (for clothing) should do the job. To avoid marring the board pieces, be sure the iron has never had impure water in the steam passages (distilled or filtered water is recommended). For each piece to flatten, place it printed side down on a piece of olefin (Tyvek or non-adhesive Pellon), and steam the back side evenly, then hold the iron (without steam) on the piece until it stops steaming. Remove the iron, and place the board piece under a weight (heavy books work well) on a flat surface until completely cool and dry.
That should fix you up.