Drought or lack of water will cause the center of cucumbers to be hollow. Hollow cucumbers will float rather than sink. You can use these cucumbers to make relish. Cucumbers want consistent, even moisture from flowering to harvest.

Here's a link to cucumber problems:
http://www.harvestwizard.com/2009/05/cucumbers_growing_problems_tro.html

Here is a link that might be useful: HarvestToTable.com

Some of mine had holes in them as well. I assumed it was a watering issue. They tasted fine and the boy liked that they looked like they had faces. Since then, I've had no problems.

Here is a link that might be useful: The Corner Yard