Like it says on the tin. Let's say a fire dragon wyrmling is directly in front of a wall of water - wall is less then a square away - and on the other side is its target. The target is 15 feet away, but again, directly in front of the dragon is a wall of water, which as per spell instructions is 30 feet long, 10 feet high, and 1 foot thick. Both targets are on the ground, and looking at the center of the water wall, but on opposite sides. The Red Dragon Wyrmling breathes in a 15 foot cone, which would have to go through the wall entirely to hit the target. Would the fire breath work?
If you are speaking of the spell Wall of Water, this case is covered in the description of the spell:
[...] fire damage is halved if the fire effect passes through the wall to reach its target
Yes it would work, but the damage is halved.