Aquatic mobs do not spawn in warm oceans in my world only. I made a copy of my world and teleported myself to a complete another warm ocean (to check if it isn't the chunks) and the bug still occurred. I also checked if the seed was the problem, but it isn't. I made my world in a version of 1.13.X (I can't remember which version was exactly, but it was after 1.13.0).I use Windows 10 home (version 1803), Java 8 (Update 211), 6GB RAM and AMD A8-7410 with AMD Radeon R5. The world size is too big to be uploaded.
EDIT: I checked if it's the mob cap
EDIT 2: My bad for not uploading the world, anyway here's the link: https://www.dropbox.com/s/y3x1oiqrxe95ux2/UNZIP%20ME.zip?dl=0