An underwater housing for containing a monitoring device includes a tubular housing body having a front end formed with an inward flange. The inward flange has a rear side surface formed with an annular groove defined by a groove-defining wall. A transparent plate is disposed fixedly within the housing body, and is located behind and adjacent to the inward flange. An annular water seal is disposed within the annular groove and in contact with the inward flange and an outer peripheral portion of the front side surface of the transparent plate so as to establish a watertight seal between the transparent plate and the housing body. An annular cover is disposed fixedly and fittingly within the inward flange, and has a rear end in contact with the front side surface of the transparent plate and in proximity to the water seal for covering the water seal.