Here are two small advantages for waiting till you get aboard.
IF you get aboard early before access to the cabins is available
IF you really like those cups they give you for pop/soda
Then buying your package upon embarkation makes sense because you will get your cup immediately. Otherwise it may be held hostage until cabin access is opened up.
To tell you the truth, I always wait till we are on the ship for another reason.
I always give my brand new, still wrapped cup away to a Mom or Dad who is also waiting in line to by a pop package for their kid. I always choose someone who looks like it will take the least amount of explaining to this little scam. If some kids want to drink pop out of the machines on me for a week; God bless 'em. If it saves a fellow cruiser $65 for the week--even better.
I have sailed the Caribbean consistently in January since 2009. I found the water in the Bahamas and St. Thomas to be tolerable, but found the water warmer in St. Maartin. I'm a California boy, though by way of Hawaii, so I have a very limited temperature range.