While I may have said that Dr. Manhattan is a being of unlimited abilities, technically he only possesses two (molecular combustion and omnikinesis), yet there is no clear end to what those strengths allow him to do. He could be in two places at once at whatever size of his choosing, pour himself a glass of water without the use of his hands (not that water is even something he requires to live), or instantaneously vaporize a human. You would not even be wrong to call him Manhattan a god as his omnipotence is no exaggeration. In addition to hands-free levitation, the blue man also has the mental capacity to see, not just the future, but ALL of time at once, meaning that he could, indeed, prevent tragedy. However, much like his precognitive vision of John F. Kennedy’s assassination, he chooses not to intervene and all for one reason which also happens to be his one true weakness.
Article source: https://www.cinemablend.com/news/2492997/dr-manhattan-vs-thanos-with-all-the-infinity-stones-could-the-strongest-watchmen-stop-marvels-mad-titan