Hi, Lucy ...
It's a remarkable thing ... teens on a ship seem to have a homing sensor to find each other.
Teen girls seem to really like working on skin cancers (getting a tan) and can find teen boys without a problem.
Have them bring a good book or two as a start.
So far as cautions go, same rules you would use at home. Curfews, check-in times and so on.
I'm a fairly liberal person, but there have been numerous times on cruise ships when the smell of "doobie" was on the air. VERY bad idea. In some countries, if you are caught with dope not even the most convincing parent or the most expensive lawyer you can find will get a kid out of the slammer.
Even if your older girls look great in bikinis (remember when we all did?) have them take a conversative one-piece for beaches ashore. No point showing off too much merchandise, y'know?
Also, they should dress conservatively for shore visits that don't involve bathing suits.
The Brittnay Spears approach is NOT recommended.
pamda -- CM First-Time Cruisers Editor