We're talking about “Minecraft” here, AKA the game's Bedrock Edition – the newer cross-platform version currently available on Xbox One, PC and smartphones.
Android, PC and Xbox One users can now toggle the option to enable new features which are still in beta testing. Right now, that includes elements from the game's upcoming aquatic update like the new Drowned mobs (think: underwater zombies), a trident weapon, new enchantments and more blocks.
Microsoft had cautioned that, as a beta, things may go wrong (so backup your save). The game does create a separate world file (with ‘EX' at the beginning of the name) to enable you to return to your prior save state should anything go badly wrong.
It's sort of like the Minecraft Java Edition's snapshots, which let you update your game client in small increments to receive new features as soon as they're coded. The latest of these includes some of the same aquatic features.