The knowledge of the noise characteristics stemming from recreational boats is scarce, and the amount of studies is too small to draw any conclusions on impact and recommend any measures. To close this gap, it is necessary to measure the underwater radiated noise (URN) from boats and estimate their source level. This chapter aims to describe the development of a methodology for source-level estimations of recreational boats based on classification society’s guidelines and test it in field trials. Three boats with different engine, propeller, and fuel types, including one electric motor, were used. Parameters such as speed, measurement depth, distance, and propagation loss estimations are varied, aiming to study their effect on the estimated source level. By combining the GPS position of the boats and sound data from a hydrophone system, successful measurements of the boats’ radiated noise were conducted, and their source levels were estimated. The source spectra of all boats showed clear tones, stemming from the rotational frequencies of the engines and propellers. Finally, a measurement guideline was developed for estimation of the source level from recreational boats that can be used for comparison with published scientific studies and environmental impact studies and as input to source models.