Marine discharges are either positively-, negatively- or neutrally-buoyant, depending on factors such as discharge temperature and salinity in relation to those of the ambient receiving waters. When planning marine discharges, appropriate assessment of the dispersion and dilution of the effluent is important for both environmental and engineering reasons. HR Wallingford has wide experience of such assessments for all types of marine discharge. In cases of either strong positive or strong negative discharge buoyancy, the density is usually dominated by either a high temperature or high salinity, with the other parameter at a similar level to that of the receiving water. In such cases, assessment is relatively straightforward. Combined facilities, such as integrated power and water plants, are in increasing use worldwide. Discharges from such facilities are typically both significantly warmer and more saline than the receiving waters, which often results in marginal positive or negative buoyancy. This can complicate assessment in several ways, which are examined in this paper. Modelling techniques are recommended that can represent satisfactorily, for example, discharges of varying buoyancy from given locations, and plumes that change from positive to negative buoyancy during the far-field dispersion phase. The relevant issues are considered with reference to recent computational modelling studies undertaken at HR Wallingford.