Archimedes' Principle - An object immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced.

Bernoulli's Principle - An increase in the speed of a fluid results in a decrease of the fluid's pressure.

Boyle's Law - The Volume of gas in a compressible container is inversely porportional to the Pressure.
PV = K   or   V = K ⁄ P   or   V = 1 ⁄ P (K)    ( K = some constant )
i.e twice the pressure produces half the volume.

Color Loss - Sea water progressively absorbs all visible color frequencies. Gone are the reds at -4m, oranges at -10m, yellows at -20m, greens at -40m, and blues at -60m. Below 60m there is a gradual fade to black.

Coriolis Effect - The deflection of fluids, relative to the solid earth beneath, as a result of the earth's oblate spheroid shape and its eastward rotation. The Coriolis Effect presents as Clockwise in the northern hemisphere, and Counter-Clockwise in the southern hemisphere. This can effect all fluids, including ocean currents & weather events.

Density - Sea water Density is dependent upon salinity, temperature, and depth (pressure). Sea water is roughly 800 times denser than air. The density of seawater at the surface varies from ranges from about 1020-1029 kg ⁄ m³. In the deep ocean it can be higher than 1050 kg ⁄ m³.

Freediving - Underwater diving without the use of any breathing apparatus.

Hard Hat - Any diving helmet, historically made of brass and copper, with small windows and open on the bottom. Surface air or some exotic gas mixture (such as Heliox) is supplied through a tube to pressurize the helmet. Because of the helmet's positive buoyancy, the diver typically wears heavy lead weight shoes to maintain a vertical posture, which is critical. They are still sometimes used for deep diving below 60 meters.

Hookah-Rig - A surface supplied air source for diving that can be dynamic (via an active air compressor) or static (via a compressed air tank). Air is supplied through a tube (thus the name) to a standard regulator. The diver usually wears a standard mask or a full face mask.

Nitrogen Narcosis - (aka Rapture of the Deep) A reversible intoxication caused by breathing Nitrogen at high pressure, usually at a partial pressure of over 400%, corresponding to a depth in the ocean of about 44 meters (132 feet) or 5 atmospheres pressure.

Oxygen Toxicity - (aka Hyperoxia) an excess of oxygen in body tissues which can be fatal at diving depths exceeding 56 meters (184 feet) where the partial pressure of Oxygen (from compressed air) exceeds 120%.
MOD (Maximum Operating Depth) Diving Tables

pH - potential of Hyrogen is defined as the negative of the base 10 logarithm of the activity of Hydrogen ions. It ranges from pH 0 (very acidic) through pH 7 (neutral) to pH 14 (very alkaline). Fresh water is neutral but seawater has a pH of 8 (slightly alkaline).

Pressure - Sea water pressure increases 0.44 psi per foot OR roughly 1 atmosphere of pressure per every 10 meters (32.8 feet). The pressure on a diver is equal to the water pressure PLUS the atmospheric pressure (e.g. 2 atmoshperes at 10 meters, 3 atmospheres at 20 meters, etc.)

Reynolds Number - The Reynolds Number is a very important dimensionless value in fluid mechanics that measures the ratio of Inertial Forces to Viscous Forces and determines if the fluid has Laminar or Turbulent flow. Re < 1,000 ⇒ Laminar flow. Re > 2,000 ⇒ Turbulent flow. Between Re = 1,000 and Re = 2,000 ⇒ Flow Transistion Zone. It is a critical value used in modeling ships, submarines, aircraft, and anything moving through a fluid.

Salinity - Sea water contains 35 PPT (Parts Per Thousand) of Sodium Chloride (NaCl), the equivalent of 35 grams of salt per liter.

SCUBA - Self Contained Underwater Breathing Apparatus

Shallow Water Blackout - A loss of consciousness (usually fatal) caused by cerebral Hypoxia when a Freediver acends and the partial pressure of Oxygen in the lungs drops suddenly to near zero.

The Bends - (aka Decompresion Sickness) The painful and very dangerous result of Nitrogen-saturated blood undergoing decreasing pressure as when rising from a deep dive OR flying within 12 hours after a moderate dive. This causes small Nitirogen bubbles to come out of solution and form in the blood stream where they accumulate in the tiny capillaries of the joints and the brain. The only course of action to alleviate the condition is to place the victim in a Recompression Chamber (aka Hyperbaric Chamber), ASAP to increase the pressure in order to shrink the bubbles, and then slowly (over 12or more hours) lower the pressure to 1 atmosphere.