It's the resort you can reach in half an hour from central Tokyo: the Hilton Tokyo Bay's summer-only Garden Pool stays open until early September and offers a leisurely day of wet 'n wild fun without the trouble of dragging a bagful of equipment down to Shonan or some other crowded beach destination near the city. Food and drinks can be ordered to your table and the entrance fee includes five hours of free parking. It sure isn't cheap, but hey: this is actually one of the more reasonably priced (and spacious) hotel pools in Tokyo.
Note that prices do not include tax.