Some ships will expect a jacket and tie for evening events. Others will not. What else you pack depends somewhat on your destination and the program aboard ship. Sportswear, shorts, swimsuits, and good shoes for walking and play aboard ship will be needed. If you are going to Alaska or somewhere else where it can be rainy and cold, pack rain gear and a very warm jacket.
Sailing is also more possible for more people now from the standpoint of seasickness.
There are two major reasons. Modern ships are now more stable, so that some of the most unpleasant motion is minimized. The other large change is the medication that is now available. Without going into great detail, Dramamine and other drugs have made a trip on a ship possible for millions who formerly would have found only misery at sea. Talk to your doctor about your best bet.