Amoxicillin could be a bad choice. First, because a LOT of people have allergies to penicillin-type antibiotics, and this could kill them. Second, from the limited shelf life, there should be other alternatives that have a more reasonable shelf life.
And if you are not familiar with the Scopoderm, read the data sheet with it. Scop is very effective a lot of the time, but also has major side effects with many people. It can also be very dangerous. I wouldn't use it on someone unless it was medically necesary, i.e., if this person isn't able to function they're going to die, unless they'd tried it ashore first. Among the complications, violent delusional pyschosis, heart problems, eye pressure problems. Or, as some of us say, "I'm on meds now. I may become an axe murderer but you don't have to worry about me being seasick, just to let you know."