Shinagawa is smaller than some of the aquariums in Tokyo, but it's full of interesting exhibits and is a great way to spend a few hours.
There are over 300 species of sea life divided into sea-surface and sea-floor exhibits. The centerpiece of the aquarium is a domed tunnel that winds through a massive tank, letting visitors experience full immersion while staying completely dry. The sheer amount and variety of fish is mind boggling.
Some of the best parts of the aquarium are the aquatic mammal exhibits. It's delightful to watch the animals play in the spotted seal observation building. There are also dolphin and sea lion shoes daily where you can watch the mammals jump and do tricks. Be careful though- the front rows will get soaked! If you're brave enough to sit up close you can buy a poncho to keep you dry.