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.
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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)