Most of the larger cruise lines offer wash and press services onboard their cruise ships for a fee. There is typically a laundry bag located inside your stateroom that you can leave for your room steward to pick up. There is a price list next to the laundry bag that lists per item amounts to either wash, wash and press or just press. You can receive perks depending on room category and cruise line loyalty levels.
If you prefer to return home with clean clothes, you are in luck. Some cruise ships also offer do-it-yourself launderettes which have 2 or 3 washers and dryers for you to use.