Officials have advised people still living up to 20 miles from the plant to leave
Radiation levels are about 1,250 times the legal limit in the ocean near Japan's stricken Fukushima 1 nuclear plant.
The levels of of iodine-131 reported several hundred metres out into the Pacific Ocean had increased ten-fold in just a few days, said the Tokyo Electric Power Co (Tepco), which operates the plant.
"This is a relatively high level," nuclear safety agency official Hidehiko Nishiyama said in a televised news conference.
Drinking 500ml of fresh water with the same concentration would expose a person to their annual safe dose, Mr Nishiyama said, but he ruled out an immediate threat to aquatic life and seafood safety.
"Generally speaking, radioactive material released into the sea will spread due to tides, so you need much more for seaweed and sea life to absorb it," Mr Nishiyama said.
He added that because iodine-131 has a half-life - the time in which half of it decays - of eight days, "by the time people eat the sea products, its amount is likely to have diminished significantly".
Tepco also reported levels of caesium-137 - which has a longer half life of about 30 years - almost 80 times the legal maximum. Scientists say both radioactive substances can cause cancer if absorbed by humans.
Officials said they would check the seawater about 20 miles (30km) off the coast for radiation and expect it to show there is no need to be concerned about any possible effect to fish.
The latest data has increased fears that one of the six reactor cores at the the site may have been cracked after a huge tsunami on March 11 knocked out the site's cooling systems.
Urgent efforts were under way to drain pools of highly radioactive water near the reactors, after several workers sustained radiation burns while installing cables as part of efforts to restore the critical cooling systems.
There have been suspected meltdowns at three of the reactors as well as hydrogen explosions and fires.
Japanese leaders defended their decision not to evacuate people from a wider area around the plant, insisting they are safe if they stay indoors, but officials said residents may want to voluntarily move to areas with better facilities.
Radioactive vapour from the plant has contaminated farm produce and dairy products in the region, leading to shipment halts in Japan as well as the United States, European Union, China and a host of other nations.