Compared to land, the ocean is a much more difficult environment to work on and in. It is opaque to most of the electromagnetic spectrum. It is constantly moving due to the effects of wind, waves, currents, and tides. It is corrosive. It is much denser then the atmosphere and its weight can produce great pressures at depth. For these and other reasons, many specialized tools and techniques have been developed to help people work on and under the water that we collectively call marine technology. This work ranges from archeological investigations to construction projects to military operations and to offshore oil & gas extraction. It includes scientists, and their support staff, who study the biology, chemistry, geology, and physics of the oceans. While the word marine implies something related to the ocean, these tools and techniques also apply to fresh bodies of water.
Below is a list of marine technologies. Clicking on any one of these will take you to a short description of the topic. Keep in mind that entire books can be written on anyone of these subjects. Where more expansive information is available on-line, a link is provided to take you to the fuller explanation. More general references can be found here.
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Spinning Mass Gyrocompass
Subsurface Drifting Buoys
Surface Supplied Air Diving