P&O Cruises outlines personal alcohol allowance and corkage rules
Eligible guests may bring a limited amount of wine or champagne aboard at initial embarkation, with different age requirements at US ports.

P&O Cruises says each guest aged 18 or older may bring up to one litre of wine or champagne when initially embarking. The minimum age rises to 21 for passengers joining a ship at a US port. The limit is applied individually to every eligible guest, rather than being shared by a travelling party.
The allowance is restricted to wine and champagne and does not cover beer, spirits or liqueurs. According to the cruise line, quantities above the permitted limit will be confiscated and discarded without compensation.
Guests may consume permitted personal alcohol in their cabins without paying corkage. A £20 charge per bottle, per occasion applies when their own wine or champagne is consumed in dining rooms, bars, restaurants or lounges.
P&O Cruises also says passengers must declare alcohol bought ashore when returning to the vessel. Those purchases will be retained by the cruise line until the end of the voyage.