Since adventure mode was introduced it had a big hole: there was no way to restart the quest from scratch when something went wrong (other than re-downloading it). With 1.15 this becomes history. There is now an option in the game menu that lets you restart. It restores the adventure world to its original state.
This option is active during adventure mode games.
The restart only works for newly created adventure worlds, because it relies on restart data that is created when you switch game mode to Adventure. Existing adventure worlds don’t have this data, and will have the option inactive until they are updated (by flipping to creative and back to adventure).
The restart position is whatever state the world is in when the mode is changed to Adventure.
And water in the bucket in the inventory is now animated :-)