All ships today have stabilizers to smooth the ride. Some cruises sail calmer waters than others. Caribbean and Alaska cruises enjoy smoother sailings 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. Tipping is up to you. Other ship personnel can be tipped at your discretion. Any state government issued drivers license or other form of picture identification should be acceptable. All others must have valid passports and necessary visas at embarkation.