Theory:

Pressure in liquids and gases
  
Important!
The pressure in a liquid or gas depends on the density and depth of the liquid or gas or the height of the gas column.
p=ρgh,whereg=9,81m/s2 - acceleration of free fall.
Water in the seas and oceans puts a lot of pressure. At a depth of ten meters, the water pressure is approximately 100 kPa, and at a depth of one kilometer, 10,000 kPa, which corresponds to a mass of 100 kg applied per square centimetre.
 
The permissible water pressure on the human body is 300 kPa.
 
Example:
To what depth can a person dive without accessories?
p=300000Paρū=1000kg/m3g10m/s2
_______________
h-?
p=ρghh=pρg
h=pρg=300000100010=30m
Answer: A person without equipment can dive to a depth of 30 meters.
  
Some interesting facts:
 
If a person has dived to a depth of 100 m, without proper suits, to survive he must spend some time in a decompression chamber.
Discover_Scuba_Diving.jpg
 
Deep-sea fish, accustomed to enormous pressure, lifted out of the water, explode. To view them, they must be placed in special cameras.
 
zz.bmp
Deep-sea fish
 
Reference:
Fizika 9. klasei/Ilgonis Vilks. - Rīga: ZvaigzneABC, 2008.-192lpp.:il.-izmantotā literatūra: 47. lpp.
Fizika 8. klasei/Gundaris Gailītis, Vija Rasmane - Rīga: ZvaigzneABC, 1997.-192lpp.:il.-izmantotā literatūra: 138. lpp.
http://www.silverspringscommunity.com/utilities/water/underdrain-system/
http://lv.wikipedia.org/wiki/Panamas_kan%C4%81ls
https://commons.wikimedia.org/w/index.php?search=scuba+divign&title=Special%3ASearch&go=Go&ns0=1&ns6=1&ns12=1&ns14=1&ns100=1&ns106=1#/media/File:Discover_Scuba_Diving_--_St._Croix,_US_Virgin_Islands.jpg