Many offer special teen and preteen scheduled activities. Cruises are a great way for families to travel and see a variety of countries and ports. Some ships also offer baby-sitting services and daycare centers. It is unlikely that you will notice any motion on a large cruise ship, even if you are sensitive to motion in automobiles or airplanes. All ships today have stabilizers to smooth the ride.
Some cruises sail calmer waters than others.
Caribbean and Alaska cruises enjoy smoother sailing's than say Bermuda. If seas do get rough, there are many preventative medications available on board. There are dozens of activities going on all over the ship every day. Relaxation is what attracts many people to cruise travel. Every ship has one or more pools that you can lay around with a good book, most have a library and several lounges where you can sit and relax by yourself or with friends.