You don't say which cruiseline you are going on but Princess and Carnival have self-service laundry room on their ships with irons and ironing boards. If it's Royal Caribbean, they offered something like 1 bag for $15, a reasonable deal. I think it would be more at a hotel if they offered it or they could direct you to the local laundrymat.
Teresa and Larry
#36-Carnival Splendor 9/16/12
7 night Mexican Riviera
#37-Allure of the Seas 11/11/12
7 night Eastern Caribbean