First a bit of silliness....no one has been sea sick in the panama canal....it's the bit getting there. Now at that time of year your chances of hitting rough water are much less than later but there are no guarantees. If you can book a cabin as near mid-ships as possibleand not too high up. If it looks like it could be choppy go to the purser or ask your cabin steward for some tablets but you will probably not need them. Brilliance is stablilised and a very steady ship and far far more stable than the IOM ferry. Seriously don't worry you'll enjoy it.....Id.
Sea sickness pills are available free of charge from guest relations. Very unlikely you will get seasick in the Caribbean though.
Bonine (Meclizine) is available onboard all ships, from either the medical facility or the guest relations desk, at no charge to guests who are feeling sea sic.