Mo Wuji's spiritual will extended towards the sea water and indeed, none of it was able to seep through the surface. Mo Wuji walked into the sea and just like how Wei Jie described it to be, the buoyancy of the sea water was still strong. The good point was that because the god energy was still of use, Mo Wuji was able to submerge into the water.
There were originally not a lot of people by the side of the God Domain Nest so when Mo Wuji submerged into the water, no one else saw him. Or even if someone were to see him, nobody would be bothered about him.
Mo Wuji continued moving towards the deep sea as he tried to use his spiritual will again. Similarly, his spiritual will was still unable to extend downwards. Even if Mo Wuji were to condense his spiritual eye, he could only see the area within 3 feet radius of him. If he were to use his spiritual will forcefully, his sea of consciousness would instantly feel a sharp pain.