I think that Sauron had to have known that Ar-Pharazon would lose. He had seen what the Valar did to Morgoth at the end of the 1st age, and Morgoth was a powerful Ainu. Ar Pharazon was just a mortal. Sauron led Ar-Pharazon to believe he could beat the Valar, but he was lying. You will note that he did not go with them to Valinor. He knew that they couldn't beat the Valar. He didn't think that they would react so violently and sink the entire island, but he did think that they would destroy the Numenoreans, and he would get what he wanted: power of Numenor, and to get back at the Numenoreans.
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For Sauron himself was filled with great fear at the wrath of the Valar, and the doom that Eru laid upon sea and land. It was greater far than aught he had looked for, hoping only for the death of the Numenoreans and the defeat of their proud king. And Sauron, sitting in his black seat in hte midst of the Temple, had laughed when he heard the trumpets of Ar-Pharazon sounding for battle; and again he had laughed when he heard the thunder of the storm; and a third time, even as he laughed at his own thought, thinking what he would do now in the world, being rid of the Edain for ever,...
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But back to the original question, Ar-Pharazon was not stupid. He was blinded by his want for long life, and he liked what Sauron was telling him. It was the sort of thing that he wanted to be true, so he believed it. I think that he should have paid attention to the stories of the Noldor being tricked by Melkor, and surely he would have heard of how Sauron tricked the Elves by making the One Ring, and this I think was his greatest blunder: he was not wary enough of Sauron and his subtle lies.