This is the kind of thing that is really a question for your table, not us, nor the rulebook.
Is your fireball...just fire? You're The Wizard in this scenario, you tell us. If it's fire, then the Paladin is invulnerable - their person takes no damage (see great answer by @Glazius).
But: Then it's also just fire. Which means you cast it underwater and it's wasted. Someone on the other side of a waterfall may also be safe, or suffer reduced damage. Cast it in a sealed room and enjoy your last moments of consciousness as the fireball greedily consumes all the oxygen and you pass out and then die.
If, on the other hand, it's not just fire, if it's more esoteric - like the extension into this universe of the fingertip of the sun god - then it may look like a fireball but it's not just fire. In which case it comes with its own set of powers and limitations, including that the Paladin is just going to have to suck it up with the rest of the targets, but there's a good argument to be made that it works underwater and through waterfalls and won't eat all your oxygen, etc.
This is the kind of worldbuilding that Dungeon World encourages and embraces - this is why the answer you seek isn't in the book - because it exists only at the table and is true at the moment you make it true.