Neuvillette had a limit to his power. So when the primordial sea water kept breaking out, it was up to him saving the citizens on Fontaine, he hadn’t realized that this would soon happen repeatedly until the last. This takes place in his last moment. He didn’t have enough energy to have set the seal off for the water. Meaning, the only thing he could do is hold back as he allowed the people of Fontaine to get to safety.