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. Hawaii-inspired 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.