Forty prisoners at Erlestoke Prison staged a sit-down protest after potentially deadly legionella bacteria was discovered in showers.
On Thursday, the Health and Safety Executive was carrying out an investigation.
The protest comes after a recent independent report described showers at the Wiltshire jail as a "disgrace".
The 20-minute protest began when contractors placed stickers on taps warning the water was unfit to drink.
The Category C prison's capacity has been increased to 426 recently, (up around 100) and during recent renovations more complaints have been made by inmates.
In July 2003 an inspection noted staff should take action against overcrowding.
A full inspection, carried out in April that year, revealed the facility was basically sound, with committed staff.
But inspectors said the jail needed a number of improvements.