Mojang yesterday announced Minecraft players on Android, Windows 10, and Xbox One can now try out a beta of the Minecraft Aquatic Update. As advertised four months ago, this update includes new swimming animations, dolphins, water biomes, and more.
Players interested in the beta are urged to check out this guide, which explains how you can access the beta on any of the Minecraft Bedrock platforms. Once all worlds are backed up, and the beta is downloaded, you'll need to check off "Use Experimental Gameplay" in the Game Settings to proceed.
Keep in mind that this beta does not include all the features coming with the Aquatic Update. Mojang mentions most features will come in future betas, and it is best not to toggle the beta off since it may cause problems with your worlds.
Java players are also getting a Snapshot today, and you can learn more about that by checking it out here.