The ferry boats are pretty big and can sail come rain or shine unless the weather is really extreme. If it's stormy or there are high winds, be prepared for the voyage to last a bit longer.
On rough days, we recommend that you bring some motion sickness medication with you and maybe even take it 20 minutes before boarding the boat if you think you might get sick. The boat staff may also provide you with Dramamine upon request. You will need to remain indoors. However, on sunny days, boats will have the sun deck open for passengers to enjoy the sun.