They get tossed around plenty too when sent through the post. Hardy fish can make it alright so long as they are bagged up correctly. For sensitive animals such as shrimp it can be useful to bag into breather bags. Since the breather bag doesn't have any air space inside the contents are jostled considerably less and the occupants are cushioned by the surrounding water.
When securing everything in your luggage it is a good idea to line the cooler holding the fish bags with a trash bag, then close it up around the fish. If one bag fails and the airline baggage handlers see the suitcase it leaking water, they will probably pull it and and not put it on the next flight.
