You will see a wide variety of daytime dress. Cunard's literature states that shorts are not acceptable in the main dining room at any time otherwise I would define daytime dress as clean and neat.
It also depends on the itinerary and where you are headed.
In the Caribbean I wear capris with a matching blouse or a summer dress.
On a transatlantic or a less than hot destination I wear slacks with a blouse and or a light jacket. Jeans are acceptable during the day. Some of the public areas can seem downright chilly so I generally add a light jacket or sweater to the wardrobe.
Sun Princess (2 times), Mississippi Queen, Dawn Princess, Coral Princess (2 times), Regal Princess, Grand Princess (2 times), Diamond Princess, Maasdam, QM2 (16 times)