Get a bucket of water, mine downwards 2 rows until you reach bedrock. Place water in one of the holes, come back up using the blocks you've collected on the opposite column from water. Once you reach land, stock up on dirt, sand and gravel blocks (have about 4 stacks to be sure). Then continue building upwards from that spot you left, until you get the message of reaching the world height limit. Once at the top position yourself facing the water miles down and free fall to it (bedrock).
In case you fail in the process, have a bed nearby to reset your spawn point.
This took me about 45 minutes or 2 hours.