From a reader:
My Grandparents are convinced that they are excused from their Sunday mass obligation because they will be out to sea on a Sunday without access to a Catholic priest. I’m worried that because they chose to go on this ship of their own free will without investigating the availability of a priest that they will fall into mortal sin if they go…your thoughts?
My thought is that if there is no priest, there is no Mass. If there is no Mass, then there is in that occasion no way to fulfill the obligation. Since no one is bound to do what is not possible, they on this occasion are not obliged to attend a Mass which is not taking place where they are.
However, when they return to shore, and if there is a Mass, and if it is a day of obligation, then they should go.
Keep in mind, dear reader, that when people are traveling, depending on the circumstances, the obligation on a day of precept is somewhat attenuated.