Is swimming your workout of choice? Many pools are part of gyms that only allow members, so finding a publicly accessible pool can be quite a hassle. Unless, of course, you’re reading my list below ↓
Tokyo Prince Hotel Sky Pool
A luxury pool with a matching price tag. But hey, you won’t be in Tokyo so often, so why not have a swim under its most representative tower, the Tokyo Tower? Keep in mind that those who have hotel reservations and pool access passes will be given priority, so check for busy dates on the website and book ahead.
Opening hours: 10:00-16:00 (opening hours may vary depending on pool access plan)
Address: 3-3-1 Shibakoen, Minato, Tokyo, Japan
Tel no.: +81 (0) 3-3432-1111
Website: http://www.princehotels.co.jp/tokyo/facility/# 
photo courtesy of hotels.com
Prince Hotel Oiso Long Beach
The Prince Hotel also has a property in Oiso in Shonan, to the south of Tokyo. The pool to this hotel is enormous, with lots of water opportunities for children as well. The pool is so close to the sea that it almost seems like you're swimming in the ocean.
Opening hours: 09:00-17:00
Address: 259-0111 Kanagawa, Nakagun, Oisomachi, Kokufu Hongo 546
Tel no.: +81 (0) 463-61-1111
Website: http://www.princehotels.co.jp/pool/oiso/
Kyōdo-no-Mori Sōgō Pool
This pool is much cheaper than the others, so don’t expect to be the only one taking a dip. Nonetheless, this pool is a good alternative to the others, since there are also many other touristic attractions nearby, such as an open-air folk museum (Kyōdo-no-MoriHakubutsukan). Remember to book ahead of time as it can get crowded during the summer.
Opening hours: vary, but usually between 10:00 and 16:30.
Address: 5-5 Yasaki-cho, Fuchu-shi, Tokyo, Japan
Tel no.: +81 (0) 42-363-8111
Website: https://www.city.fuchu.tokyo.jp/shisetu/supotu/puru/sogopu-ru.html