Check out the discussion threads on cruise critics website. On one cruise line (Princess?) / Itinerary in 2016 we learned they would offer a midweek special for $25 laundry bag service. For us it was worth it to pack less and spurge for laundry service.
Last year on a Viking River cruise (plus other self travel) we did carry on luggage. That meant doing some hand wash of underwear, socks and some lightweight tops for me. We splurged for laundry service for some of my husband’s polo shirts ($6.50 each).