Yachtsman is very good - it's steak house. If they would rather not spend a ton of money, but still want good food, Capt. Grills at YC is also excellent. As PP has mentioned, Cape May is a big seafood buffet at BC (right next to YC).
If they aren't on the dining plan, I would also recommend looking at bluezoo at the Dolphin (right next to YC) and Il Mulino (Italian) and Kimono's (sushi) at the Swan. Swan/Dolphin has, arguably, some of the best restaurants on property.
We also like Trattoria al Forno at BWI (but Il Mulino has better Italian), though I'd actually recommend walking over to the Boardwalk after the cruise to see the nighttime entertainment or even perhaps go to one of the clubs (Atlantic Dance Hall and JellyRolls) or go to AbracadaBAR for some unique drinks. They will also find Ample Hills at BWI which has some of the best ice cream on property.