The use of biocidal antifouling (AF) paints is the most common way to prevent fouling on leisure boat hulls. The main aim of this thesis was to investigate the pathways through which AF biocides, past and present, may reach the environment through their use on leisure boats and to improve the risk assessment of biocidal AF paints intended for amateur use. The work presented focuses mainly on the Baltic Sea, with emphasis on regulation and risk assessment procedures in Sweden. A new method was developed for the quantification of nowadays banned organotin compounds (OTCs) such as tributyltin (TBT) in paint flakes (paper I). OTCs were detected in hull paint scrapings from three countries around the Baltic Sea. Thus, historic layers of organotin paint on leisure boats may constitute as sources of TBT to the marine environment. Total tin was identified as an indicator for the presence of OTCs on boat hulls, allowing for quicker identification of vessels in need of remediation. Nowadays, most AF paints tend to contain high amounts of copper (Cu) and zinc (Zn). The use of AF paints was shown to cause exceedance of guideline values for these two metals in soil, sediment and water in various investigated marinas (papers II and IV). The pollution of boatyard soil was linked to hull maintenance activities carried out over unprotected ground (paper II). AF paints were also found to impact both the concentration and speciation of dissolved Cu and Zn in two Baltic Sea marinas, with increased concentrations as well as an increased proportion of bioavailable species as a function of an increased number of moored boats (paper IV). A new method utilizing X-Ray Fluorescence (XRF) was used to derive the release rates of Cu and Zn in the field for five commercially available AF paints for amateur use (paper III). Salinity and paint properties were found to be important parameters affecting the release. The in situ release rates were also found to exceed those derived with current standardized release rate methods. Given the high release rates, none of the studied paints should have been approved for the Swedish market. This finding likely explains the exceedance of guideline values for dissolved Cu and Zn in investigated Baltic Sea marinas (paper IV). In conclusion, there is a need for caution when authorizing new biocides as the phasing out of banned substances can be a lengthy process due to their continued presence in historic paint layers. Additionally, paint-specific release rates determined under conditions reflecting the intended use of the product should be used for a more realistic environmental risk assessment of AF paints.