Hi Dannyr,
Don't worry...its not really a problem. They get plenty of water from the fruit and veg they eat. My pigs don't generally drink that much. Only if they've had a lot of dried food or if the weather is warm.
How do you put the water in the cage? Is it in a bottle? Do they pay if attention when investigated their territory? If they do i wouldn't worry, when they need to drink they will. And remember, you can't watch them all the time...they will drink if they need to...
Zoe.