Tsuruga is a unique place. It’s the first town over the border from Kansai in Hokuriku. It is a beautiful little city and worth spending a day exploring and it has a great supersento. It’s more than just a supersento, though. It’s got a whole indoor-outdoor pool attached to it. I really loved this place. The staff were pleasant. The onsen from two springs is fed into pools of various temperatures. The springs both have a good onsen feel to them. The bathing area is small and there is not much selection, but it is a rural supersento and that is expected. Still it has more selection than some places I have been. The landscaping in the rotenburo area was green and very relaxing. The view is astounding as the building is high up on a hill overlooking the city, a freeway, and the Japan Sea. The pool area is great too. Super fun for playing around in. It does not have any slides, but it is not just a rectangular pool. The outdoor lounging area was great too. That astounding view makes it what it is.
The hot spring here was originally found when a tunnel was being drilled through the mountains hence the name Tsuruga Tunnel Onsen. Take the tourist bus from the station or drive cause it would be a pretty tough walk. The tourist information office has discount tickets.
Update August 2019: Returned here with my wife and daughter. We enjoyed swimming around the pool but not being too noisy as the pool seems to be there for old people to exercise in more so than family fun. It was a great time and a lot of fun. I also was able to enjoy the other bath on the mountain side. Not much of a view from the outdoor bath, but the design was outstanding. Highly recommend.