Seawater desalination in the Arabian Gulf and the Red Sea is the reliable source of water supply to the population of Kuwait, Saudi Arabia, Bahrain, Qatar, and the United Arab Emirates. If desalination plants were to operate along the coasts of these and climate semi-enclosed Arabian marginal seas, then the additional loss of water and returned brine waste due to the plant's water production would increase the salinity. A mathematical model is presented to calculate the impact of desalination plants on the salinity within a semi-enclosed sea of simple geometry. Due to the exponential sensitivity to the plant's location and its water production capacity, the effect of seawater desalination at the northern Arabian Gulf or Red Sea is found to be more severe.