I just use a large hiking pack, and/or a diving bag made by Cressi, both are long enough to fit long fins (I took 3 pairs) in. The fins, if they are carbon or fiberglass, are put in a sheath made of 5-7mm corrugated plastic and some tape (nothing too fancy) and packed between lots of clothes and wetsuit as padding. If you can't get a bag big enough some airlines, I would call and ask first, will let you take them carry on and leave them at the front with the stewardess' luggage.
The water is currently 23 and will go up to 27 until. Im Canadian and always wear a 3mm if its warmer than 18 so it depends on your tolerance to the cold. For most people a 3mm is fine though, unless you are really sensitive to getting cold.