P and O cruises are an entirely separate company (owned by Carnival) to p and o ferries.
I have been on 15 cruises, love them. Mainly Cunard (quite formal), P and O (less formal) and Princess - informal, but you can dress up if you want.
If you can stretch to it, I’d recommend an outside cabin or balcony, especially for the fiords.
Choose you deck carefully, always go for one that has cabins above and below. You don’t want to be above the disco!!
Mid ships is more stable than forward or aft, a consideration if you may be sea sick.
We have NEVER been I’ll on board in any way. Just follow all the rules re hand washing etc. and you’ll be fine.
We currently have 4 cruises lined up, having had at least 6 cancelled. We love them as the places come to you! And we like sailing out of Southampton as my DH came from there and my DS now lives there.
Loads more tips I could give you, any specifics let me know.