No offense Paddy, but for just an overnighter, Okehampton to Princetown by bus wouldn't really be that simple - 2 buses needed, the Tavistock/Princetown bus is not that frequent so it'd take quite a while - even if there were connections at the times you needed them (unlikely IMO).
Time on/waiting for buses is time not walking!
Waterwise, you'll be fine. Even for a hilltop camp, it's never that far from a source.
PS If you want less bus time and to get straight onto the moor, then get off at the Dartmoor Inn*(Lydford Turning)Last Year -We left the car there and bussed back to it whenI guided a hillwalking newbie (older lady) from there to Okehampton via Yes Tor for her first moorland part of her Michael Mary Line Pilgramage walk
* could still go SSE to Tavy cleave via a few nice tors
Edited: 26/03/2012 at 13:10