The idea of retaining water is good, but the deployment is not great
He should have tried to direct the water perfectly over the dragon, creating a barrier channel and a holdback right after the dragon and eventually locking up, drown and freezing the dragon
But we have to believe that the author just wanted to show a bit of intelligence from the MC (but as the author is not himself very intelligent or the artist has poorly represented the scene)
he ended up doing a new overkill skill that spends a lot of mana, instead
According to me and my basic knowledge in physics
all the story with resistance or weakness to water is biased from the start, because in the end what matters is more or less the contact surface in the case of non-magic liquid water.
Moreover from the physical point of view and in view of the heat described there cannot be complete contact between the dragon and the water, because the heat evaporates the water and creates an air barrier.
(Except perhaps in the case of sufficient pressure, but in that case he would have created a jet of water and cut the dragon with it, frankly it would have been a better smart way to kill him)