How can I handle expansion drainage from water heater?
This sounds like an ideal situation for a condensate pump. Those are miniature sump pumps with a built-in reservoir. You drain your seepage into this, and the pump pumps it up and away when the reservoir reaches a certain level. Just make sure you get one with the capacity to pump as high as you need to to reach the outside, a drain, or whereever you want to dispose of the water.
These also usually have a 12V NO and NC switch that you could use to wire in an alarm if you wanted to.