C172 Flight Instructor with student reported losing engine power during a cross country training flight resulting in an off-airport landing. Upon inspection; maintenance discovered water in the fuel strainer suggesting fuel contamination.

Narrative

My student and I were conducting a cross-country training flight from ZZZ1 to ZZZ on Day 0. We took off from ZZZ1 around XA:30 in Aircraft X (Cessna 172N); performed one touch-and-go in the pattern; and then proceeded westbound at a cruising altitude of 4;500 feet towards ZZZ. Approximately 20 NM north-northwest of ZZZ2 and 20 NM northeast of ZZZ; we began our descent from 4;500 feet to set up for ZZZ and to stay clear of the clouds in the area. At around 1;700 feet; our engine sputtered; and the RPM dropped from 2;300 to 1;500. We were gradually losing power and were unable to restore engine performance.I took control of the aircraft and performed the emergency checklist. The fuel selector valve was already set to 'Both;' the mixture was already on fully rich; and the propeller was still spinning. However; I could not regain power; and our altitude was around 1;400 feet. I [advised ATC] and executed a landing on a field 90 degrees to our right; heading east.After landing; I secured the aircraft; confirmed that both occupants were uninjured; and verified there was no damage to the aircraft or property. The oil level was at 7 quarts; with 11 gallons of fuel remaining in the left tank and 15 gallons in the right tank.The maintenance found water in the fuel strainer. They assumed that it was due to heavy rain or fuel contamination from the fuel truck.This area is encountering mid afternoon thunderstorms bringing in heavy rain daily. We should be extra cautious during pre-flight to check for water. Even though we thoroughly checked and sump the fuel; water was still found after landing and from inspection.Cessna 172N only has three sumps in total. I can now see why later models have 13 sumps.