Skyhawk 172/Cutlass 172
Flight Crew Private
Flight Crew Last 90 Days 6; Flight Crew Total 120; Flight Crew Type 17
Aircraft Equipment Problem Critical; Inflight Event / Encounter Fuel Issue
Flight Crew Diverted
Aircraft
Before takeoff [on my return flight from a short cross country] I did a full preflight; checking fuel and for water in both the tanks and gascolator.  When I reached the north side of a [frequent VFR Reporting Point] I called Tower and started my descent to traffic pattern altitude.  The engine began to run rough so I applied carb heat with no change.  I switched the fuel tank selector from both to right and after no change; returned to both.  I advised Tower I had to land the airplane; picked a nice grass field and landed safely.  Upon landing; I checked the gascolator and tanks.  I found water in both.  I secured the plane; consulted an A & P; purged the fuel system of water and ran the engine to verify the aircraft was airworthy.  I then measured and prepared the field to ensure a safe takeoff and flew the plane back to base without incident.
The pilot of a C172 returning to base suffered a rough running engine which forced a safely conducted off airport landing.  When he sumped the tanks and gascolator he discovered water in both.  After appropriate ameliorative; testing and planning measures the flight back to base was conducted without further incident.