Sometimes the biome slider on the first page of the customized world setting says All but generates only Ocean.

Steps to recreate:
1. Start creating a customized world, go to the first page of options.
2. Drag the biome slider to ocean.
3. Drag it slowly back just to where is says all.
4. Create world. It will be all ocean.
5. Exit world, start recreating it.
6. Go to the first page of customized world options. The biome slider will now say Ocean.