Fouling from the biomass present in seawater is a major drawback in the operation of marine reverse osmosis plants. Much in-plant work has been carried out to reduce or alleviate the problems, and there are some tools that can be used for assessing the probability of water causing biofouling. However, there is not a generalised tool, and more specifically, there is no standard indicator of the biofouling potential in waters loaded with biomass. An indicator such as silt density index (SDI) is often used, although it cannot give precise information about biomass. Other indicators such as the modified fouling index (MFI) have been developed to improve the information acquired from the saline water. Adenosine triphosphate (ATP) has been suggested as an indicator of biomass contents, therefore indicating biofouling potential. When subjected to the light and luciferine-luciferase is added, ATP produces luminescence, which can be detected and measured in relative luminescence units (RLU). Water samples can be taken with the assistance of a fouling biomonitor and later analysed by photoluminescence. Although there is some published information about measurements in slightly saline waters that is not the case in seawater. The purpose of the paper is to provide results of measurements of ATP and other indicators of biomass content made on seawater samples in the Atlantic Ocean. The first issue when measuring marine waters is to take care of the saline effect, e.g. the interference in ATP measurements due to the high salinity of seawaters. We have developed some modifications in the analytical procedure to avoid the saline interference, published elsewhere. With that modified procedure, we have carried out some measurements in different locations and conditions. Both laboratory and field tests were performed. The obvious advantage in the former is a more stable process where sampling is more reliable. The field tests were performed on feed-water before and after passing through sand filters, which gave us the opportunity to analyse the accumulation of biomass in those filters. Another test is related to a comparison between water extracted from an open sea intake, as compared to seawater drawn from a beach well. In the latter case, the ATP contents are lower, as expected, indicating lower biomass contents and biofouling potential. A first evaluation of the development of ATP along time has also been carried out, in order to determine the biofouling potential rate (BPR), with preliminary results showing little growth, most probably because of the quality of feed water, which is filtered through the beach well.