Java Edition:Ender Dragon in Boat

From 15w41a to 15w47c (1.9 snapshots), the Ender dragon was able to enter a boat. In 15w49a (1.9 snapshot), this bug was fixed.^[1]

Obtaining

You can get the Ender dragon in the boat anywhere during flight, but from the exit end portal is the easiest. Since it will come to the pedestal periodically, you can put the boat on top of the bedrock pillar of that portal. Once the Ender dragon is in the boat, the player cannot enter the boat because its hitbox is much larger. However, the player can be in the boat when the Ender dragon enters. This will allow the player to steer the ender dragon around but also take lots of damage. It's possible, though, to stay alive with healing, such as enchanted golden apples and diamond armor enchanted with Protection IV. The player can also get multiple Ender dragons in boats by getting multiple ender dragons. The ender dragon can be in oak, spruce, birch, jungle, acacia, and dark oak boats.

Transportation Between Dimensions

With 1.21 Pre-Release 1 changing portals to allow riding entities to travel through them, this also includes the ender dragon.

Whenever the dragon exits the end it will destroy the blocks around it so be careful to allocate space accordingly, about 17x17 area.

Nether

For the nether this can be simply done by placing a Nether Portal in the end or by creating a portal around the ender dragon in the overworld.

Ender dragon with a nether portal around it
The same ender dragon now in the nether

Overworld

For the overworld you can either be done by creating a portal since 14w28a (1.8 Snapshots) using end portals to create a portal or by placing a Nether Portal in the end and then creating another nether portal in the nether to move it to the overworld.