Fogging is a well-known issue that affects cameras that are placed inside a waterproof housing, when humid air is trapped inside the housing and condenses to form water vapour or droplets on the inside of the camera lens cover. This often occurs when the sealed housing is exposed to cooler temperatures outside the housing, such as when you place the camera underwater (snorkelling, diving, surfing, etc). Anti-fog inserts can be placed inside the sealed housing to help absorb the moisture and avoid ruining your footage.
If you are planning a snorkelling or diving holiday to countries or regions that have high humidity, don't forget to pack some Anti-Fog inserts. For example; Coastal areas up in Queensland that have very hot and muggy weather will be VERY humid and Anti-Fog inserts will almost certainly be required. You will definitely require them for Fiji. Areas that have dry air with Low humidity (such as the Snowy Mountains) rarely suffer from Fogging.
The problem can occur in all GoPro cameras that can be removed from the housing. Models that are factory sealed into the housing are NOT affected by fogging (GoPro HERO 2014, GoPro HERO+, GoPro HERO+ LCD, GoPro Session4, GoPro Session5, GoPro HERO5 Black, GoPro HERO6, GoPro HERO 2018 all DO NOT need them). The exception to this is if you place any of those cameras into an EXTRA Dive housing, such as the GoPro Super-Suit Dive Housing or similar. In that case, Anti-Fog inserts should be used.