Build up to block 244, one under the block height limit. Then, remove all blocks under the top block like in the screenshot.

Place a boat like in the screenshot, get in, and fall through the block.
Oh, but that's not the bug...

Once you fall back to the ground, get out of the boat and...

You will be back up at the block where you started.

Note: Momentum is capped, as you are falling your speed will fluctuate if you start going too fast.
Note: You cannot place a block or another boat on the block you fell from.

What the heck is going on?
Will post short video.