We were on the Serena earlier this year from the Med into the Atlantic down to the Canaries and back again and to be quite honest I doubt if you will feel seasick at all!!! It is a huge ship and hardly moves in the water. However if you think you will be, it would be much cheaper to take tablets with you (we always carry Stugeron tablets available from Boots but have never had to take them.) The injections are very expensive apparently around 50 pounds a time. Tips if you do feel queazy are to keep in the middle of the ship and go as low as you can to minimize the movement. I'm sure you will be fine.
I've been researching Costa as I'm considering a winter cruise on the Serena. She like most ships has a medical centre onboard. The injection for sea sickness is usually always available. I had to get one on the Lirica last month, not because I was sea sick but because I had a chest infection and couldn't stop being sick. Had be vomitting for over 20 hours and stopped within 15 minutes of having the shot. I'd only have it as a last resort after trying the usual remedies such as tablets or the wrist bands. However if you are feeling awful, then it is certainly worth a try. Not sure how much they will charge you. MSC kindly didn't charge me at all. (Think they must have felt sorry for me!