So apart from the Drowned I have not had any other aquatic mobs spawn in 1.14.3 so whatever was resolved in 1.14.2 to get them spawning has broken again in this update like it did in 1.14 and I fear it hasn't been addressed as it isn't in the 1.14.4 pre release 1 fixes list.
Environment
All I can currently state is that the issue was resolved in 1.14.2 and aquatic mobs spawned fine on my system. But in 1.14.3 the issue has reverted back to 1.14 where they are not spawning at all.
Linked issues
duplicates 1
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Duplicates MC-148613 (fixed in 1.14.4).
I have the same problem, but only with the world, transferred from old versions. There's no such thing in the new worlds. (sorry for my English if there were errors)