Built in 2018, the building is quite modern and clean, and has a unique layout starting you up on the top floor with a view of the Joetsu coastline, and then working its way back down to the 1st floor.
Along the way, you'll see not only fish, dolphins, beluga whales, and jellyfish, but also a projection mapping of the four seasons, and the biggest population of Magellanic penguins in Japan.
Like most aquariums, there is a dolphin show, but I found it to be cramped in comparison to Marinepia's stadium area. Instead, I'd recommend trying to catch the feeding time for the beluga whales, when you can see the staff doing some up close training with them.
The penguin enclosure is impressive, as you'll be walking among the birds (just be careful not to get too close), and there are several areas below their enclosure where you can watch them swim. The penguin area, as well as the carp pond and the touch pool, are all outside, so your experience may depend on the weather.
Joetsu Aquarium Umigatari (limited English-language page)
