Sourcing energy from renewable sources is currently a key theme in modern society. Consequently, the pace of development of these emerging technologies is likely to increase in the near future, particularly in marine renewables. However, the environmental and ecological impact of many of these new developments in the marine environment is largely unknown. My thesis has focused on one unknown area of interaction; the potential effect of tidal-stream devices on marine mammals. Collision risk is often cited as a key concern. Therefore, my premise was - for marine mammals to avoid a collision with a marine renewable device (assuming they are on a collision course) they must first detect the device. It is well understood that marine mammals use sound and hearing as their primary sense for communication, foraging, navigation and predator avoidance, so it is highly likely that the primary cue for device detection will be acoustic. However, it is not known how operational marine renewable devices might modify the acoustic landscape in these areas, or whether they will be audible to marine mammals in time to alert them to the presence of devices. It has been suggested that the high level of natural and anthropogenic background noise in tidal-stream areas may mask (drown out) the signal of the tidal devices. The acoustic characteristics of underwater noise in shallow coastal waters are currently not well known. My thesis adds data to this knowledge gap by measuring and mapping underwater noise levels in tidal-stream areas.