To ensure you never need a "fix" again, follow these rules when creating your flat colony.
What if you already spent 10 hours building a castle on a broken flat world, and you just realized there is no Iron ore within 500 blocks? You don't need to restart. You need the Post-Gen Patch. colony survival flat world seed fix
Note: You must have Admin privileges or be playing in Single Player. To ensure you never need a "fix" again,
If your flat world is underwater, use the Global Heightmap Adjuster: colony survival flat world seed fix
/terraform raise 10
This raises every block within a 50-block radius of you by 10 levels. Repeat until your head is above water. Then use /terraform flatten to smooth the edges.