We finally had that chance about a week or so ago when we were trying to find a water park to visit for the first time this summer, and also baby's first time swimming. So after some searching, I came across the Fuji Marine Pool (富士マリンプール).
Their facilities met all of our criteria: river pool, kids' pool, babies wearing swimming diapers OK. The place also has a neat little wave pool, several waterslides (some small speed slides and some tube slides), a concession stand/food shop, and lots of space to spread out towels and mats (which people do, so you'll want to get there early to find a decent spot, let alone in the shade). And Mt. Fuji in the background (not visible in the picture above though). Of course.
They also have a concession stand with various typical cheap food options (curry, yakisoba, ice cream, etc.).
We enjoyed spending our morning here with baby - definitely recommended!
When: Open the last Saturday in June until the first Saturday in September, 9am-5pm
Adults 500 yen
Elementary/Junior High students 300 yen
Infants over age 1 100 yen
Lockers are 200 yen but you get 100 yen back when you leave (bring the locker card key upstairs as you leave).
How to get there:
25 minute walk from Yoshiwara station (吉原駅) or 5 minutes by car/taxi. Or take bus from Yoshiwara station going toward Numazu station or Higashitagonoura station (沼津駅または東田子の浦駅) and get off at Ono Shinden (大野新田). 3 minute walk from there to the pool.
275-9 Tanaka Shinden
Fuji, Shizuoka 417-0843
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Parking: Some. Get there early.
Official Website: http://www.fuji-kousya.jp/mp/