Just been on the Ventura and our cabin had both three pin plugs and american type plug sockets. Hope this helps.
Every P&O ship I have been on has 220 volt sockets in the caibins, not many and sometimes hard to find.
If you look on the P&O website you can read under questions "Cabins on all ships are fitted with three pin standard British style sockets and the current on board is 220 volts AC at 60 cycles, so all British style hairdryers will operate normally. However, we do provide hairdryers in the cabins. Additionally all bathrooms have razor sockets with 220 / 110 volt switchable sockets."
Costa ships have botheuropean and americn plug and I beleive so does the Queen Mary .