In April we sailed from Miami to Barcelona - which entailed 5 consecutive days at sea. It was the most entertaining and relaxing cruise we have been on (except for an exceptional cruise up the Nile River).
I also was worried about being bored - but it was anything but. It can be relaxing - laying around the pool. But it can also be engrossing. We took Spanish language classes, ballroom dancing, ping pong tournaments, Pilates classes, attended lectures, drank the free champagne at the art auctions, visited the exotic dance clubs (no, wait,,,that was New Orleans) and enjoyed meeting the passengers and people watching.
The days at sea can be whatever u want them to be. I wrote a report on the five days at sea earlier. If you go back a couple of pages you can find it under "My Transatlantic Crossing on the Brilliance of the Seas".