https://en.wikipedia.org/wiki/Atomic_bombings_of_Hiroshima_a...:

“…many U.S. officials and scientists argued that a demonstration would sacrifice the shock value of the atomic attack, and the Japanese could deny the atomic bomb was lethal, making the mission less likely to produce surrender. Allied prisoners of war might be moved to the demonstration site and be killed by the bomb. They also worried that the bomb might be a dud since the Trinity test was of a stationary device, not an air-dropped bomb. In addition, although more bombs were in production, only two would be available at the start of August, and they cost billions of dollars, so using one for a demonstration would be expensive.”

After the two bombs, there was an attempted coup to prevent the Emperor from surrendering.

And you think bombs at sea would scare them?

The bombs barely performed their intended effect being dropped on cities. Even after Nagasaki the Japanese military argued they could continue to fight from underground bunkers.