LOW Sea Level

[Meledosia]

I've been experimenting with "Customized Worlds" by misode trying to get this:

1. An underground of only about 6 blocks
2. Sealevel at 6 blocks
3. Multiple biomes so I can have hills etc.

No luck so far. I can set the sea level low enough, but whenever I do it ends up not generating. If I try to adjust the land lower the pack won't load. Can anyone tell me if this is possible and if so, how to achieve it? Super Flat will get me the tiny underground but without any oceans or hills. The scale I'm working on makes building terrain by hand unrealistic.