Underwater archaeology is the study of past human life, behaviours and cultures using the physical remains found in salt or fresh water or buried beneath water-logged sediment. It is most often considered as a branch of maritime archaeology. Due to the difficulties of accessing underwater sites, the application of archaeology to underwater sites emerged from the skills and tools developed by salvagers, and underwater archaeology initially struggled to establish itself as proper archaeological research.

Underwater archaeological sites consist of wrecks (shipwrecks or aircraft); the remains of structures created in water (such as crannogs, bridges or harbours); refuse or debris sites where people disposed of their waste, garbage and other items by dumping into the water; or places where people once lived, that have been subsequently covered by water due to rising sea levels or other phenomena.

There are many reasons why underwater archaeology can make a significant contribution to our knowledge of the past. Some individual shipwrecks are of significant historical importance either because of the magnitude of loss of life (such as the Titanic) , or circumstances of loss (Housatonic was the first vessel in history sunk by an enemy submarine^[5]). Shipwrecks (such as The Mary Rose) can also be important for archaeology because they can form a kind of accidental time capsule, preserving an assemblage of human artifacts at the moment in time when the ship was lost.

Sometimes it is not the wrecking of the ship that is important, but the fact that we have access to the remains of it, especially where the vessel was of major importance and significance in the history of science and engineering (or warfare), due to being the first of its type of vessel. The development of submarines, for example, can be traced via underwater archaeological research, via the Hunley which was the first submarine to sink an enemy ship (Hunley also had unique construction details not found in previous vessels and was one of the only historic warships ever raised intact), the Resurgam II, the first powered submarine^[8], and Holland 5, which provides insight into the development of submarines in the British Navy^[9];

Underwater archaeology is often complementary to archaeological research on dry sites because materials are preserved differently under water than on dry sites on land. In anaerobic, cold and dark conditions underneath waterlogged sediments, organics, such as plants, leather, fabric and wood may be preserved as on Hunley and Mary Rose. These materials may still have evidence of how they were worked, such as tool marks on the surface of wood. This evidence can provide new insights into ancient crafts, cultures and lifestyles.

Underwater archaeology is not just about shipwrecks. Changes in sea-level, because of local seismic events, such as the earthquakes that devastated Port Royal^[10] and Alexandria, or more widespread climatic or changes on a continental scale mean that some sites of human occupation that were once on dry land are now submerged. At the end of the last ice age the North Sea was a great plain, and anthropological material, as well as the remains of animals such as mammoths are sometimes recovered by trawlers. Also, because human societies have always made use of water, sometimes the remains of structures that these societies built underwater still exist (such as the foundations of crannogs, bridges and harbours) when traces on dry land have been lost.