There is less water on Mars compared to Earth. Most of the water known is in the cryosphere (permafrost and polar caps). There is no liquid water. There is only a small amount of water vapor in the thin atmosphere.

The conditions on the planet's surface do not support the long-term existence of liquid water. The average atmospheric pressure and temperature are far too low, which freezes water. However, it seems Mars once had liquid water flowing on the surface. This would make large areas like Earth's oceans.

There are a number of signs of water on or under the surface, now or in the distant past. These include stream beds, polar caps, spectroscopic measurement, eroded craters. Also, there are minerals which are often  formed when there is liquid water (such as goethite), grey, crystalline hematite, phyllosilicates, opal, and sulfate.

The Mars flybys such as Viking, Mars Odyssey, Mars Global Surveyor, Mars Express, and the Mars Reconnaissance Orbiter had cameras.  They took pictures of what seem to be ancient lakes, ancient river valleys, and widespread glaciation. An orbiting Gamma Ray Spectrometer found ice just under the surface of much of the planet. Also, radar studies found ice that were thought to be glaciers. The Phoenix lander showed ice on Mars when it was landing. Phoenix also showed ice melting, snow falling, and even saw drops of liquid water.

A recent report says Martian dark streaks on the surface were affected by water.

Water found by probes 

Many Mars flybys and probes have found evidence of water on planet Mars. Mariner 9 found water erosion and deposition, weather fronts and fogs. Viking 2 landed on Mars in its winter season and found frost. The Mars Global Surveyor, which could detect ancient water, found that Mars has been dry for a very long time. On December 6, 2006 NASA released photos of two craters called Terra Sirenum and Centauri Montes that showed liquid water at one point in 1999 and 2001. Pathfinder found that water existed before on Mars. Pathfinder confirmed that where it landed it is too cold for liquid water to be. However, water could be in its liquid state if it were mixed with various salts. Pathfinder also found evidence of clouds and maybe fog present in Mars. Mars Odyssey found much more evidence for water on Mars. The pictures taken by Odyssey confirmed that the ground is filled with ice. In July 2003, at a conference in California, it was said that the Gamma Ray Spectrometer (GRS) onboard the Mars Odyssey had found large amounts of water over large areas of Mars. Mars has enough ice just below the surface to fill Lake Michigan twice.

Liquid water found on Mars 
On August 4, 2011, NASA said they found seasonal changes. These changes were found in gullies near crater rims on the Southern hemisphere. This shows that there is salty water flowing and then evaporating. This leaves some sort of residue.

The Mars Ocean Hypothesis is a hypothesis that nearly a third of the surface of Mars was once covered by an ocean earlier in its history. The ocean, which is called Oceanus Borealis, would have filled the Vastitas Borealis basin in the northern hemisphere. The Vastitas Borealis is 4–5 km (2.5–3 miles) miles below the planetary elevation. Oceanus Borealis dried up 3.8 billion years ago. Early Mars would need a warmer climate and thicker atmosphere to let liquid water stay at the surface.

References

Other websites 

 YouTube: Liquid Flowing Water Discovered on Mars
 High resolution images of Mars—detailed observation of water signs
 The Case of the Missing Mars Water 
 Wikinews articles about water on Mars

Mars