One of the earliest parks to open their outdoor pools for the summer season is Numakage Park in Urawa, Saitama. Information adapted from Saitama with Kids facebook page (with permission):
The outdoor pools in NUMAKAGE PARK have been opening for weekends since July 2nd. They will open daily from this Saturday the 16th of July.
July 2nd to August 15th: 9am to 5.30pm.
August 15th to 31st: 9am to 5pm.
430 yen for adults, 210 for primary to high school students, preschoolers and below free. *However, children still in nappies / diapers are not allowed use the pools*
10 minute walk from JR Saikyo Line Musashi-Urawa station.
Parking: for 130 cars. FREE.
Satnav / GPS: phone number 048-861-9955
8 pools including 1 with waves and 2 suitable for young children (who are out of nappies). The children’s pools have a maximum of 50 cm of water and both have slides.
Other: Numakage park also has an all year round indoor pool.