The mobs aren't spawning in their default biome and the ones I enabled too. Only Mantarays and sharks spawn

Same, But I also have seahorse and crab spawning. Basically all of the ocean creatures spawn for me. Nothing on land. Using the Fabric 1.21.8 version of the mod.

You need to regenerate your world, either by creating a new one or by deleting the regions in your save folder. Be careful not to delete the region where your builds are located.

Problem solved like that for me.

Regards