If you already booked THAT cabin, and you feel you must have a view of the water, you only have 2 choices- 1) call them back and pay for the "upgrade" to an unobstructed view or 2)call them and change your reservation to a TBA (To be Assigned) or Guarantee cabin which means you are confirmed to have at least the obstructed view you already have or the may upgrade you to a better view. Since you did not say when you were cruising I can't really say if either option would be available to you since you may be too close to your sail date for them to make a booking change without a penalty (paying for the upgrade probably would not include a last minute change charge.
The obstructed view will still allow light into the window, but if you feel you must also be able to see the view, you should try for the upgrade.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007