Japan already imports close to 60% of its food, so importing a little more probably won't be noticed so much.
Food that is grown in the open near the reactors is being and will be affected. However, Japan's breadbasket for farm products like potatoes and dairy products, the island of Hokkaido, is largely unaffected, as are the vast majority of farming areas west of Tokyo. Also, many winter foods are grown in greenhouses, so just about any greenhouse-grown product outside of Fukushima will be OK .
The farmers in Fukushima Prefecture will undoubtedly be hurt substantially, and farmers in Ibaraki Prefecture will also feel a lot of pain from this, but farmers in most other prefectures likely will not be affected on a large scale.
Prickly territorial waters issues are nothing new to Japan-- there have been ongoing spats with Russia, South Korea, and most recently China about islands and the waters around them. But Japan's own territorial waters are actually quite extensive-- they cover an area many times larger than the country's land area. The areas shown in light blue on this map show Japanese exclusive economic zones.
Also, most of the ocean to the east of these zones is international.
The fisherman in eastern Fukushima, as well as parts of Ibaraki and Miyagi, may have farther to go for fishing, or give up the trade altogether. I can also envision Iwate and eastern Aomori prefectures being affected as well (especially Iwate, with all that debris from the tsunami in the water). But fishermen in other parts of the country probably won't be affected so much.