Twenty-one or over, and both of you
Everybody in the room has to be twenty-one or over, whoever wrote the booking. A photo document may be asked for at the door, and is never copied.
One age applies here and it applies to every person present. Twenty-one or over, whoever booked and whoever else is in the room when she arrives. A couple is not a single guest for this purpose; it is two people, and the line is read against each of them separately.
What happens at the door
She may ask to see a photo document from either of you. She glances at it and gives it straight back. Nothing is copied, scanned, photographed or written down, and no detail from it is kept anywhere afterwards. If she cannot settle the question of who is in the room, she leaves before anything begins, and the fee for the reserved time stands.
Why a passport is usually already to hand
This is the one part of the rule that suits cruise passengers. Two people the night before a sailing are carrying documents anyway, because the morning will want them, so the request that feels awkward on an ordinary trip is close to trivial on this one.
Booking on behalf of somebody else
You can write the thread for the two of you. What you cannot do is book for a third person who is not part of the conversation, or write in a name that is not yours. Both people in the room must have agreed to the booking, which she checks when she arrives rather than assuming from a thread she never saw.
The rest of the statement
This site is written for adults and carries an age gate at the entrance for that reason. The age statement sets out the whole of it, and the terms restate the same rule in colder language.