Fresh, often colder and less dense water enters salty, often warmer, denser water as a surface or subsurface plume, eddy or lens. Bottom sediment changes drastically due to sediment load and sediment and current flows increase. Increased current flow can affect amphibious craft and swimmer navigation. Reefs predominate in tropical areas and are often unsurveyed. All three can affect approaches taken by amphibious craft, swimmers and torpedoes, and can adversely affect active sonar.
Most shipwrecks occur near shore.
These hulks can present a hazard to navigation, but a bottomed sub can merge her signature with a shipwreck to escape detection. Light is produced when plankton organisms are disturbed with no correlation to weather or sea state. It can be observed from the surface to 35 feet. The result is enhanced nighttime visual detectability of bow waves and wakes of swimmers, periscopes and surfaced and submerged craft. A ship's wake may be visible for 6 nautical miles.