Most tree types are willing to grow into water blocks, given they have a pocket of air to be planted within. Dark Oak trees don't consider it disqualifying, either, but it seems like the method that generates the trunk is unwilling to overwrite water blocks, leaving the branches and leaves floating, disconnected.

How to Reproduce:

1. Locate a large body of water with a reasonably flat floor in a normal world, or generate a Superflat world using the Water World preset.
2. Build an air pocket to plant saplings in. Make sure the roof of the pocket is a block that trees can grow through (i.e. logs or leaves)
3. Plant four Dark Oak saplings in a square in the pocket, then apply bonemeal.
4. Observe the structure of the tree that this creates.

Screenshot 1 shows a bugged Dark Oak tree, as well as an Acacia tree and a giant Jungle tree that grew correctly in similar conditions. Screenshot 2 shows a non-bugged Dark Oak tree grown in an identical planter, but out of water.

Screenshot 3 shows a bugged Dark Oak tree being grown from a larger air pocket. Screenshots 4 and 5 show a Dark Oak tree, bugged from being grown into a waterfall.