Open Water Swimming Near Tokyo (or b/w Tokyo & Narita)

I'm going to be in the Tokyo region on business in a few weeks and will have my last day free prior to a 5 p.m. flight out of Narita. Though I think I may go to Ueno Park to visit some museums, I may also be in the mood for an open water swim. Has anyone done any in the Tokyo region? Thanks for any input, Jack