[deadboi]

In order to produce a flame you require a combustible substance and an oxidizer.

In the case of a candle the combustible substance is the wax and the oxidizer is the surrounding oxygen. Thus as long as you have both you will have a continuous flame.

In the case of an underwater cutting torch both a combustible substance and an oxidizer must be provided by hoses leading to the torch. Hydrogen and acetylene are commonly used combustible gases and the oxidizer is oxygen. The gas must be supplied at a high enough pressure to overcome the pressure exerted by the water to maintain a continuous bubble around the flame.

So it could be that the fire based spells and items that work underwater provide both their own combustible substance and oxidizer contained within a bubble while ones that don't must rely on materials that are already present. The same idea could be used in the case of the frost brand in that the magic effect is contained within a bubble.