The waterpark at Toshimaen としまえん is a bit rundown & pricey but quite fun! Just a couple of minutes on foot from the Oedo Subway station with the same name (30 min. from Roppongi / 20 min. from Shinjuku) or right out of the Seibu train station also with the same name. Once you get in the park, follow big signs that says "pool" in katakana for a few more minutes. The water park is all the way at the back.
It's 4000 yen for the water park with wave pools (very small waves) and Hydropolis water slides. It costs another 600 yen each for the day if you want to use the water slides using single/double tubes.
Just 14 out of the 26 water slides were open, 8 closed/open slides plus 6 open slides for tubes from 80 to 140 meters long. The other slides were under maintenance. On a Saturday afternoon, it wasn't so busy. We had to line up for 5 to 30 minutes for each slide.
There wasn't a line for the giant inflatable slide called, the Hippo which is also included in the extra 600 yen you pay for the slides using tubes. Maybe it's meant for kids, but it turned out to be quite fast and exciting!
The slides using tubes had shorter queues, less than 10 minutes, especially the ones out back. The shorter slides are faster and more exciting. The double tube (for 2 people) is more fun since you might not always be facing forward on the single tube (for 1 person). Even though the admission is already quite steep, we think it's definitely worthwhile to pay the extra 600 since you can go down the slides many more times because the queues are way shorter.
The locker costs 400 yen plus a 100-yen deposit but you can open and lock it as many times as you'd like. The locker area doesn't seem to have individual changing rooms so it might be good to bring a towel to cover up. There are hairdryers though.