Our daughters have had them done on our last 4 cruises. We have had them done on the ship as well as at port (St. Maartin). It is typically $2 per braid, the problem off the ship is they will put, not even kidding, 20 - 25 braids in their heads. On the ship (3 times) they never braided more than 10 to 11 braids.
The braids are tight and it really just depends on your daughters head. We usually cruise with family friends and some kids just have a softer scalp and will cry throughout the entire process and sometimes complain later that their head is throbbing. My girls have pretty tough heads and tell me that it's painful (no tears) but they want to do it again each year!!
They do share combs with each girl, I'm sure they will not be insulted if you bring your own comb.
I hope this was helpful!