Beyond the "under-the-hood" code, 1.13.0.4 addressed several immersion-breaking visual bugs:
Watch this breakdown of the 1.13.0.4 beta to see the hidden features and technical changes in action:
Ultimately, 1.13.0.4 was not designed to be a "major" update but a "maintenance" beta. It paved the way for the stable release of the Update Aquatic features—such as dolphins, shipwrecks, and the conduit—on mobile devices by ensuring the game engine could handle the increased complexity of the new ocean biomes.