Embarkation in Venice was good and our cabin was available on arrival. Viking arranged special assistance for my wife and this worked well in both directions.
There is no doubt that the Viking Sea is a really lovely ship. There are no facilities for children and, in fact, under 18s are discouraged. It is bright, airy and extremely comfortable with a real feeling of space. Public areas (including walkways) are wide and it is very easy to move around the vessel. There are plenty of areas around the ship to relax in. There is no dedicated library but every 'lounge space' had bookshelves with a selection of fiction and non fiction material (even the pool area). We did not try the Spa but other passengers spoke highly of it.
The cabin (it is called a stateroom) is generous. A king sized bed, two comfortable chairs, a writing desk with chair and an occasional table. The bathroom is a joy. It is much more roomy than we are used to with very good sized shower - the heated floor is a rather nice touch. There is a large wardrobe (we had to ask our cabin steward for more hangers but they arrived very quickly) and more than enough other storage spaces.
The Deluxe Stateroom is the same size as the entry priced cabin but includes a coffee maker (but not tea?), dressing gowns and slippers, a fridge with soft drinks (replenished free each day), binoculars and priority booking for specialist restaurants.