Yes you will have to register with for their IT services and the cost might not be cheap but I should imagine that might not be your major concern.
Best ring P&O and check what is available and where. i.e. on some ships the Wireless Network is not ship wide but in certain areas only. Access tends to be slow and very expensive unless you buy a package and even then it cannot be descrobed as cheap. As long as the ship has sateillite contact then you can connect. This is not always the case but in most areas of the world this is not a problem. Check with P&O they will often get the ship computer person to e-mail you.
If you want to use the internet you have to go into the wireless areas i.e bars, however as i thought it was stupid that it was a brand new ship and didn't have wireless in the rooms, I asked if there was any way of getting internet in the cabin and the computer support person kindly came and fitted a wire from the tv to my laptop. But as one person has already mentioned, it's worth giving P&O a ring and have a chat to the internet person as soon as you get on the ship, because some things are first come first serve.
I hope you get a better deal from Ventura than Thomsons offer £1 PER MINUTE IN WINTER AND £3 PER MIN IN SUMMER-- THE CAPTIVE SYNDROME