Created a new world on 1.13 and after a few days ALL aquatic passive mobs have vanished entirely.
I note a number of other people have reported the same issue yet they have all been marked as resolved, when the bug is still there.

I understand mobs will despawn, under the right  criteria, but they should also spawn which they do not. Not even the squid.  And this is the same anywhere in the world I visit.

Two things that may or may not have contributed to this issue.

i) I built an underwater kelp farm before the fish disappeared

ii) I captured some tropical fish and put them in an aquarium.

Even the Hermitcraft server is experiencing the same issue

I hope you will actually  resolve this bug as it is just one of a plethora of bugs in 1.13.
Whilst I realise bugs are inevitable, the shear quantity of them in 1.13 is beyond belief. I hope this is not the sign of things to come.....