But, the tanks are leaking. According to Salon.com:
"The latest leak comes from one of the site’s 1,000 tanks, about 500 yards inland, Tepco said. Workers discovered puddles of radioactive water near the tank on Monday. Further checks revealed that the 1,000-ton capacity vessel, thought to be nearly full, only contained 700 tons, with the remainder having almost certainly leaked out.
There had been concerns raised among some experts over the durability of the tanks. Mr. Ono said that Tepco had assumed the tanks would last at least five years, but the latest leak comes less than two years after the company started installing the storage vessels at the site to deal with the growing amounts of runoff."
No matter where the radioactive waste goes, it is a disaster, but it could get to the ocean, and that is probably the worst that could happen. It's not just that there is a bunch of radioactive water on the loose, but it's that the destroyed power plant is generating radioactive water and can't be stopped from continuing to do so, or so I think the issue is.
It's a big ocean, but not that big.