We made reservations and paid for the Undersea Expedition over two months ago. A couple of weeks prior to the trip, I received a call that we would need to change the time of the reservation due to a "scheduling conflict." No worries...we were able to secure a time that would work within our itinerary. When we boarded the vessel, there were no seats left and we had to leave the vessel. There were no more expeditions scheduled for that day and we were leaving the following morning. While staff were as helpful and sympathetic as they could be, the expedition was oversold by some other booking entity (not Viator) to accommodate a cruise ship full of travelers. We were not able to do this activity, which was the one we were really looking forward to. Viator was very helpful, and we got a full refund, but I had to do all of the leg work to make that happen.