hi, yes i agree when you give them there veggies just leave them wet that way they will take in water,but it could be that they are comming out to drink when no ones around or when your asleep,mostly piggys wont come out to explore straight away when they are in a new inviroment,they could stay there for a few hrs to days to a week sometimes longer depending!it depend on them when they relax and get use to things that they will venture out and explore,
they will be able to smell the water so will know where it is,but if your really worried then as well as doing the veg thing you could put a dish of food and water near to where they are,
I'm sure in a few days they will be out exploring there new home,