I like Keria Izakaya at Cuppage, love the vibes. Needs booking, only does dinner, run by a japanese obasan, sometimes she mans the phone.
https://www.facebook.com/p/Kerria-Japanese-Restaurant-100063608290246/
There is another izakaya nearby at the basement called Dosukoi x Donpachi but it is more a watering hole for japanese, decent food as well.
Wasabi Tei is another japanese eatery that I frequent when I dun feel like spending too much. No GST no Svc charge, very limited seats, abit like Midnight Diner: Tokyo Stories style, the current owner is friendly, the previous one more than decade back lashed at customers, but people kept going back.
I dun really jiak seafood per se if u r referring to those 活海鮮 style Sg restaurants..
** I dun jiak omakase, so the food is not as finessed , not haute cuisine level, so depending on what u are looking for **