Physical exhaustion is very much a part of sea sickness. Additional, typical of cruisers in general, culinary and alcoholic indulgence also lower the threshhold for developing sea sickness. So, although it is ok to partake in activities, the abundance of food, and some of those wonderful drinks with umbrellas and sold in souvenir glasses, know your limits, and you will lessen the chances of developing sea sickness.
Enjoy a cruise!
John Burns, RPh