I used to work for Starboard/Greyhound myself - for several years! And my company MK Cruise Ship Recruitment were their main hiring partners for the UK and Europe. I know the HR Managers and DM's very well!
I could write a book about my experiences onboard and with Starboard (in fact I have!) but i don't have the time here to answer all your questions.
However, I will say that for your first job on a cruise ship - you wouldn't go too far wrong working for them. They're a decent company to work for and in general while not being the best paid position, if you're looking for a good time with lots of free time in port, then the giftshops is a great department to work in. Go for it!
By the way, if you're interested, I have a copy of the actual interview questions that Starboard will ask you!
Best of luck.