Possible Scenarios
During the search, most thought that the three prisoners had failed to make it across the San Francisco Bay, and drowned in the water on the way to the shore. There were a flurry of reasons as to why no one thought the prisoners survived their trip through the bay, one being that the water was incredibly cold, making it very difficult to navigate with the freezing water splashing all over you.
A few other reasons are that the current of the bay actually goes outward, into the middle of the ocean, in addition to their being many sharks waiting for a nice meal. These three reasons combined to make people believe that there was no chance that the prisoners successfully escaped Alcatraz.