Why did the Baker test contaminate ships with radioactive seawater?

The Baker test, conducted on July 25, 1946, at Bikini atoll, resulted in the contamination of naval vessels because it was the world’s first underwater atomic explosion.1

The 23-kiloton device was detonated at a depth of 90 feet (27 meters) within the lagoon. The explosion generated a massive column of water, approximately 2,200 feet (670 meters) in diameter, which rose to a height of one mile. As this column of water collapsed, it created an expanding spray that engulfed about half of the target fleet. Because the explosion occurred underwater, the water itself became intensely radioactive, saturating the ships with lethal levels of radiation that made them unsafe for inspection for several days. This persistent contamination was so severe that it ultimately led to the cancellation of a planned third test, known as "Charlie".2