For those travelling with their BlueROV2, Blue Robotics strongly suggests you remove enclosure vent plugs during shipment by air or road, to allow the enclosures to change internal pressure and reduce stress on the penetrators. Just remember where you packed them for re-installation when you arrive.
Also remember to take extra desiccant (silica gel) bags to put into the electronics enclosure at your destination, to prevent dome fogging during the dive.
Finally, about flying with ROV batteries: it is our understanding that you can fly with one Blue Robotics 18Ahr battery packed in the ROV (obviously, not connected) but you cannot take any 18Ahr spares in the cabin (they ship as hazardous goods cargo only). Up to two HobbyKing 8Ahr or 10Ahr batteries can be taken as spares in carry-on luggage, only with prior approval of your airline. Each carrier has their own rules about lithium batteries, so call before you arrive at the airport.
The Fifish V6 can ship as is, as long as the ROV tether plug is in place to prevent electrical shorting. The V6 has a 97.2 Watt-hour battery capacity that is under the 100Watt hour limit for passenger plane carry-ons. It is always wise to take a copy of the technical specifications with you, in case you are asked about the equipment.
To calculate the capacity of your batteries: Power capacity = Voltage * Current * Time e.g. Blue Robotics: 266.4 Watt-hour = 14.8 Volts * 18 Amp-hour