Water productivity (WP) metrics have proven useful in comparing production efficiency of various crops. Recently, it has also been proposed to facilitate more equitable allocation of scarce freshwater resources between irrigated agriculture and fisheries. Parameterizing water productivity metrics, however, proves to be very difficult in the real world of inadequate data and complex aquatic ecosystems, and is usually impossible to calculate for exploitation strategies, such as fisheries, that harvest products, but do not in and of themselves reduce the natural resource base from which those products were derived. In special cases, marginal water productivity (the variation in production for each unit variation in available water) can be estimated, but the complexity of its use under-values the social, ecological and economical importance of fisheries and so cannot be recommended for making inter-sectoral comparisons.