@Kataoka "Buri" is hamachi? Saba (mackerel) is a separate piece of fish that you want to cook?
The 3-day marinade was maybe too long. All that salt (shio miso) maybe drew out too much moisture from your fish.
Fish is sometimes sold, frozen, with a miso marinade (thick) already on it. So, I'm assuming it's OK to store it like that. I would defrost it before cooking.
Personally, I would eat the hamachi raw, all of it.
Edited to add: Actually, maybe the 3-day marinade should have been OK. Try Googling "misoyaki butterfish". The recipes call for a long marinade. Not sure whether buri and butterfish are exactly comparable, though. Hamachi seems more delicate.