These days it’s pretty easy to find regularly updated information on the levels of radiation in Japan. http://atmc.jp/
This is my go to site for regularly updated levels by prefecture. There was a scare with the Tokyo drinking water, but levels have since returned to normal. http://metropolis.co.jp/quake/quake-201 ... formation/
I haven’t heard about any other problems regarding drinking water safety outside of the immediately affected area.
I’d like to say not to worry. The Western press has had a heyday with all the radiation paranoia. My family and I are not planning on leaving any time soon- but we live way far way from the problem. I wouldn’t make any snap decisions until you get your placement. Most of Japan is still unaffected by the Fukushima reactors. And the ocean is a very big place. I certainly wouldn’t willingly expose my kids to tainted drinking water or unsafe living situations, and I’m sure you wouldn’t do that either. (I mean the program is great, but not *that* great.) But most of Japan is perfectly safe for kids.