I suggest you book shows online if you want a choice of seating times. Otherwise you have to take what's left, and some can sell out leaving people in a stand-by status. Some likely are already sold out for online bookings since they hold back a certain number of seats for those who want to wait and book aboard. They only hold back a small amount of seating for shipboard booking, unless online reservations didn't max out.
As for spa treatments - I've never heard of them being hard to book aboard the ship, and given how they push them from the moment you walk aboard I am sure you will be fine in waiting for those reservations.