Depending on the place you are going you might or might not need certain things. But there are many items everyone should take under consideration, while preparing for a sailing trip.

Breathable wind/rain jacket – is a must have, this small softshell jackets are easy to pack and might bring this additional comfort. 

A pair of Good Polarized Sun Glasses even in cloudy weather, sun reflection from water is pretty strong. Protect your eyes, and you will feel much more comfortable.

A good, portable camera. Space is a luxusty on a yacht. And you definitly do not want your nice and expensive camera to be destroyed falling down from the tabel on a big wave. Consider getting a portable camera like for example Go Pro camera with a hand or chest mount set.

Water bottle and some drinkable water or other liquids. Very important, if not the most important point. Sooner or later you will get thirsty. Getting a botter of water might be a life saver in case someone forget to fill the water tanks or get sick on board. 

A good Waterproof bag or backpack 

Extra pair of cloth, sealed in plastic bag or in waterproof backpack

Suncream, if you are heading to warmer locations. 

Enjoy your sailing.