[Agema]

You could argue that flame brands have magic fire like the astral fires spell. Or that it superheats the water around the target to boil them. Whereas your average fire spell uses magic to conjure a normal fire, which would get instantly snuffed out underwater.

Technically frost brands wouldn't be much use underwater either if you used the same logic to ban fire brands, because the minute you drew it it would freeze the water and you'd end up clubbing people with a large, heavy, unwieldy lump of ice.