Freshwater spawns normally (somebody said something about manatees not spawning, im unsure). But orcas, squids, tropical fish, pufferfish, and all the like dont spawn.

assumedly because salt water is bugged

I saw this in testing mob spawning in singleplayer as well, and on my server oceans are quite empty of mobs (besides jellyfish? For some reason those were spawning)

I have gone out of my way to test this out, and it is indeed a thing. I observed both aquatic life in lakes and rivers and the ocean.
it seems theres alot less life in freshwater then previous versions? but thats probably not true. fish still spawn in lakes and rivers so theres no worries there.

i flew around slowly in the ocean in creative mode using night vision to inspect the sea life easier. absolutely nothing even after 30 mins of loading new chunks and staying in some for a long time. observed this originally in a minecraft server

using 4.0.8-beta TFC,

MODLIST:
jei-1.21.1-neoforge-19.25.0.322
Patchouli-1.21.1-92-NEOFORGE
tfc_support_indicator-1.0.3+mc1.21.1
TFCVesselTooltip-NeoForge-1.21.1-2.0

I am guessing this has to do with us using SpawnPlacementTypes.IN_WATER for all of our ocean entity spawners. Need to add our own spawn placement type for saltwater, and another than supports both fresh and river water