No guarantees on weather, but most ships will give you as smooth a ride as possible. Are you subjct to getting carsick? If yes you are likely to experience some motion/seasickness, if no, then sea motion is not likely to bother you. My daughter swears by Seabands (elastic wrist bracelets) for my two granchildren who tend to get carsick.
Most cruisers love the days at sea. Sleep 'til whenever, leisurely meals, lazy days around the pool, plenty of shipboard activities provided by entertainment staff, bingo, art auctions, casinos, trivia, games at the pool, spas, gyms, (golf depending on ship), cards , internet, library, movies, dance lessons, tours, cooking lessons, wine tastings,
sunbathing, just to touch on a few things to do(or not do).
Relax, enjoy, unwind.