Cessna Citation Sovereign (C680)
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Flight Instructor
Flight Crew Last 90 Days 90; Flight Crew Total 8500; Flight Crew Type 1763
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Executed Go Around / Missed Approach; Flight Crew Diverted; Flight Crew Took Evasive Action; General Maintenance Action
Aircraft
Crossing the final approach fix we descended down to 860 feet MSL. At level off we got a caution obstacle from the GPWS and at the same time a low altitude caution from Renton Tower. They advised us we were at 660 feet; but our indicated was 860 feet.  As we executed the missed approach ATC gave us the altimeter setting of 29.85 which was exactly what we had set. Once on with Seattle Departure we climbed to 4;200 feet and requested vectors to BFI for an ILS. We also asked ATC to keep an eye on our altitude because we believed that we had an altimetry error and we were IMC. Our altimeters were jumping and not steady during the ILS. Upon landing the aircraft was written up with the maintenance discrepancy. A Cessna Technician purged the system and noticed water coming out of the lines. The system was tested and returned to service.
CE680 Captain reports descending to 860 feet during a non precision approach to RNT and receiving  a caution obstacle from the GPWS and at the same time a low altitude caution from Renton Tower is issued. A go-around is initiated and the flight diverts to BFI for an ILS. Maintenance finds water in the pitot static system.