Most major cruise ships have complementary meeting facilities, rooms and reception areas. Larger ships have theaters which can accommodate over 1000 people. Short of bingo days and nightly shows, they tend to go under utilized. Most presenters opt for the smaller venues on cruise ships. Most larger ship have 3 to 4 meeting rooms which can accommodate 50-100 people.
The onboard staff can help you with room setup, internet access, monitors, VCRs, DVD players, microphones, podiums, projectors, etc. Many of the ships can even do photos and video production for you (although usually at an additional charge). Your seminar can actually turn into a product by the end of the cruise.
If you want to dine together privately or have a private reception, meeting facilities for this can often be arranged usually at no charge for larger groups. A few years ago we had a suite while cruising in Europe which came with a butler. We had several small parties in our suite and we just told him how many people were coming so they could prepare enough food. We would regularly have 15-20 guests that would stop by for appetizers and champagne before dinner. The butler would serve everything and the room would be all cleaned up by the time we got back from dinner. What could be nicer.
And then there are specialty features you might want to take advantage of. Have a mini-golf tournament. Go rock climbing as a confidence building exercise. Some ships have zip lines as well. What about private exercise classes. And then there are the spa treatments. The later aren’t free, but I guarantee someone in your group will want to try them.