Unable to answer "What are the opening hours here?" – Cromaris (Seafood Shop) – https://osm.org/way/115848664 via StreetComplete 58.2:

House with No. 12 on it looks like empty, with housing attached to the right.
https://cromaris.com/en/consumer/cromaris-fishmongers/
mentions
Zaton Fishmonger
Zatonskih pomoraca 10,  Zaton – Nin
but that address does not look like a shop either. 
Locals asked at the small store near by send you to the main road towards Zadar to buy fish.

Attached photo(s):
https://westnordost.de/p/231028.jpg