From the P&O Cruises website:
You only have to cruise with us once to receive complimentary membership to our loyalty programme – The Portunus Club. The club has three different tiers starting with Ruby, then Sapphire and the pinnacle tier is Gold. Each tier has both on board and shoreside benefits associated with it. You’ll earn points for every night you cruise with us and, the more points you earn, the higher your tier and the greater the benefits. Essentially the more you cruise with us, the more rewards you’ll receive.
Your membership will automatically be activated after your first cruise with us, at which point we’ll send you a detailed membership pack, your membership card and your points statement which details the number of points you have earned and the tier that you are in.