Yes they should let you take them around the X rays, but don't seal the lid until afterwards otherwise you will just have to unpackage the box i presume you are using? Also they should be taken as hand luggage (quite obviously), perhaps on your lap for the flight, I'd recommend one of those "freezer boxes" made of polystyrene and then put the bag inside that so it keeps warm and try not to open it too much. I don't know how long the flight is but they will not need that much water but a lot of air (especially since your air will not be pure O2 like fish stores). So really a minimal amount of water maybe 1/4 of the bag the rest air. Seal over the corners of the bag with cellotape so the fish don't get stuck and die (ie so its rounded at the bottom).