I just got back from the 3/2 Magic cruise...I got cornrows done in St. Maarten on the beach (Orient Bay) by a local woman. Make sure that you agree on a price BEFORE they start braiding...we agreed on $30...she did it right on the beach...and it took about an hour. She used foil on the ends...it's cheaper...but rubber bands are better. If they have rubber bands, request those, but chances are...since most of them do not have much money, they will go for the cheapest thing, and that's foil. She did a very good job...however...
I got the rest of my hair braided in
Castaway Cay. The ladies that do it there come from a nearby island to do the braiding, and they use rubber bands (which are better and easier to take out). I was much more impressed with the braids that I got at Castaway Cay. Plus, since Castaway Cay is the last port, your braids are fresh for when you go home...and won't get all yucky from salt water, sand, and wind. Also, the lady even took the foil off of my St. Maarten braids, and put rubber bands on instead so they'd stay in longer...how nice of her. I was charged $1/braid there.
They do do the hair braiding on the ship now (on sea days only, normally by the Goofy pool)...which would probably be your best bet as you don't have to waste precious time at the ports getting it done. I suggest waiting until the last sea day. Also, it's first come, first serve, and many times it's very difficult to jump in there (as they don't take any appointments).
Hope this helps!
Mickybean0816