Job 9:8 (WEB)
8 He alone stretches out the heavens, and treads on the waves of the sea.
God "alone" treads on the water. Jesus walks on the water in the Gospels (Matthew 14:24-27, Mark 6:47-50, John 6:16-20), and this verse is a prophecy of that event, teaching that Jesus is God.
The book of Job is a poetic book, and in context, Job is describing God's greatness by appealing to creation:
Job 9:4-10 (WEB)
4 God who is wise in heart, and mighty in strength: who has hardened himself against him, and prospered?
5 He removes the mountains, and they don't know it, when he overturns them in his anger.
6 He shakes the earth out of its place. Its pillars tremble.
7 He commands the sun, and it doesn't rise, and seals up the stars.
8 He alone stretches out the heavens, and treads on the waves of the sea.
9 He makes the Bear, Orion, and the Pleiades, and the rooms of the south.
10 He does great things past finding out; yes, marvelous things without number.
Above, in verse 8, Job does not mean that he has seen God walking on the water. He is being poetic about God's sovereignty over the bodies of water on earth. It is a severe over-reading of this passage to try to make it teach that if someone walks on the water, they must be God, or it must be a signal that they are God - nothing like that is stated at all.
However, does that prevent it from being applied devotionally or prophetically to Jesus walking on the water in the Gospels? Not necessarily, but no New Testament author makes that connection. Of course, God is able to empower people to walk on water. Such a miracle is a very small feat for God, who made the entire universe.
In the account of Jesus walking on water, Jesus empowers Peter to walk on the water as well, and he appears to successfully walk on the water for awhile, before beginning to sink:
Matthew 14:28-31
28 And Peter answered him and said, Lord, if it be thou, bid me come unto thee on the water.
29 And he said, Come. And when Peter was come down out of the ship, he walked on the water, to go to Jesus.
30 But when he saw the wind boisterous, he was afraid; and beginning to sink, he cried, saying, Lord, save me.
31 And immediately Jesus stretched forth his hand, and caught him, and said unto him, O thou of little faith, wherefore didst thou doubt?
When Jesus told Peter to walk on the water, He enabled Peter to do so. In the same way, God enabled Jesus to walk on the water. That in no way violates Job 9:8, because Job 9:8 is not teaching that if someone walks on water, they are God, or that it is a signal that they are God.
And, consider the general tenor of the Trinitarian argument. They are maintaining that God is essentially hinting that Jesus is God by having Him walk on water here, based on a specific interpretation of a passage in the book of Job, which is not cited in the New Testament. This is not a stable foundation for a major doctrine concerning God in the Bible. These kinds of verses are appealed to only because the actual doctrine that is being argued for is not stated explicitly at all, ever, in the Bible.