You don't need to bring cash for the placeholders. You reserve them now through the
DCL app, and it'll charge the $250 deposit to your credit card. It's risk-free (other than letting Disney hold onto your money), because you can cancel it and get a refund if you decide not to take another cruise down the line. You don't even pick the cruise on board...just reserve a placeholder for the discount, and then you can go home and check your schedules for what you'd like to do next.
As for seasickness, some people swear you should start taking meds before you get on board, but then how do you know if you need it? We never needed it on our first cruise, and never really noticed the wave action until after the cruise, walking around Epcot on flat land and not experiencing the waves anymore.
