This paper represents the beginning of a reference data base for the long term assessment and control of environmental impacts on the coral reef habitat of the Balhaf Bay, Gulf of Aden, following the development of an industrial complex on the bay. Present results reveal a high surface water temperature in summer with the tendency for relatively low temperature in the winter months. Bottom water temperature undergoes significant seasonal variation, with the annual difference at the two studied stations found to be up to 13.9 °C in the deep station (D), and 11 °C between August and September in the shallow station (F). Winter salinity variation was found not to exceed 0.5 g/l (measurements between 35.7 and 36.2 g/l), though salinity did vary seasonally: increasing at the beginning of the summer monsoon from 35.7 g/l to reach an annual maximum in this region (36.5 g/l) in the first days of July. The general lack of difference in the index of salinity for the studied depths is worth noting. Whatever the reason, such results call for more detailed studies of the habitats under reference.