When the test was held, the ship was outside the "danger zone" established by the U.S. military and 100 miles form the test site. But the bomb tested was twice as powerful as expected (and 1,000 times the strength of the bomb used in Hiroshima). The fisherman were exposed to nuclear radiation as "death ashes" fell from the sky for several hours while they tried to retrieve their fishing gear from the sea.
Upon returning to Tokyo, the fisherman complained of burns, headaches, pain their eyes and other symptoms of radiation poisoning. The chief radio operator died six months later from blood and liver damage. The rest of the 23-man crew were hospitalized for a year.
The Daigo Fukuryu tragedy sparked a global anti-nuke movement and inspired a series of famous monster movies in Japan -- starting with "Godzilla" which came out later that year.
Here's some photos of the Daigo Fukuryu and the nuclear museum in Tokyo.
The Daigo Fukuryu in its current home in southeast Tokyo.
Atomic tuna. In addition to the Daigo Fukuryu, 856 smaller Japanese boats were affected by the blast and ensuing radioactive rain. In all, 485 tons of tuna had to be discarded because of nuclear contamination from the Bikini blasts.