So there is a bug where cod and salmon won't spawn in water world flatworld preset but its a deep ocean I only see dolphins, drowned and squid.
MCPE-238114
Fish don't spawn in the Water World flat preset
Resolved
Awaiting Response
1
Fish spawn within 12-32 blocks of the player. In Bedrock Edition, this means that the solid block block of the bottom of a continuous column of source water must be within 12-32 blocks of the player. Does that explain your issue?