Time Out says
Tokyo Summerland is the embodiment of the old adage that one man's paradise is another man's hell. The concept (as if you hadn't guessed) at this huge theme park to the far west of the capital is summer, and providing the opportunity to experience the hot season all year round is its purpose. The indoor pool is notorious (see the oft-embedded video below), though the outdoor one is more bearable, with 650 metres of flowing water – dubbed 'The Great Journey' – as its star attraction. Beyond the pool, the roller coasters are many and varied, but so are the people. Probably best to go on a quiet weekday morning offseason if you have the luxury to do so.
Tokyo Summerland indoor pool