Thomson Calypso is adults onl.
Try P&O Arcadia its 18 and over all the time, others do one now and then ,a Saga cruise would do but are smaller ships.
Saga, and 2 P&O ships. Azamara, Seabourn, Silverseas, OCeania do not have childrens facilities but that does not mean you won't get ankle biters on boar.
Saga, P&O Artemis and P&O Ocean.
Anything on the arcadi.
The P&O adult only ships are Artemis and Arcadia, not Oceana.