I would like to know how similar Mars water is to Earth water. Can we survive on Mars with Martian water without any major health issues etc?
Fundamentally, water is water. In its purest form, it is the same anywhere, except perhaps for the isotopes. However, one of the wonderful things about water is the fact that it's a good solvent, and in fact has many things in it that aren't water. For instance, one could not survive off of ocean water: we humans require fresh water.
The one potential difference is in the isotope of the water. This is a rather complex concept, basically some water is heavier than other water, despite having the same chemistry. It turns out that Mars has a higher concentration of heavy water than Earth does. However, it is still within reason to us humans to use. It would make fusion easier, but that's another matter entirely.
The water that was discovered on Mars recently is unique. It is believed to be a very salty water; in its natural form humans could not drink it. However, it can go through a desalinization process, where the salt is removed, and in theory it should be drinkable. Basically, the water needs to evaporate, then re-cool.
As was said in the press conference, the best source of water for astronauts would be in the ice caps, which are similar to Earth's ice caps: frozen water on the surface, which would simply require melting to use, along with a small purification process.
Bottom line, there is water on Mars, and through some purification process, it can be made for human consumption.
This discussion on quora talks about deuterium abundance on Mars:
Deuterium occurs naturally on Earth in water as 1 in 6,400 hydrogen atoms or 1 part in 3,200 by weight. On Mars it is one deuterium for every 1,284 hydrogens.
So high enough to be noticeable in a lab (for example Mars water is about 700 ppm denser than Earth water under equivalent conditions) but not enough to be relevant for drinking.
Mars lacks just about everything else for creating sustainable life. The most damning -- and the one that everyone seems to forget -- Mars has no magnetosphere or atmosphere like we do. The solar wind is not at all deflected around Mars and gamma radiation destroys DNA. It is just too small to have either -- not enough mass to hold onto an atmosphere gravitationally and it apparently also does not have a sufficient core to create a magnetosphere.
The water would not only be unsuitable for life as we know it -- it would also be irradiated to toxic levels. On the upside, however -- it should be relatively virus free.