If your friends are really set on booking one, I would go ahead and do it, with a clear understanding of the cancellation period. When I got to my pre-cruise hotel in Piraeus, I would check the 5 day weather forecast on whatever weather app you have on your phone. If it looks to be too chilly, cancel then, or as soon as you board. Your sea day is between Santorini and Valletta, and Santorini would be too late to cancel if the limit is 48 hours. EM