Cruisin'

My parents are planning a cruise later this year and, since they've never gone on a cruise before, they would appreciate advice. So if you have any to give, please let me know! It can be which line is best or worst, any good or bad experiences you had (directly related to the cruise), what ships are best, what areas are best (they're planning Caribbean), how much cash they should take along (if any), if they can take bottled water along, anything at all.