"Since 'tis certain that Earth and Jupiter have their Water and
Clouds, there is no reason why the other Planets should be without
them. I can't say that they are exactly of the same nature with our
Water; but that they should be liquid their use requires, as their
beauty does that they be clear. This Water of ours, in Jupiter or
Saturn, would be frozen up instantly by reason of the vast distance
of the Sun. Every Planet therefore must have its own Waters of such
a temper not liable to Frost."

Christian Huygens, 1698

This delightful quote was issued today by NASA, in the context of discussion of rivers of Methane and even "mud" on Titan. I would recommend all budding space scientists to take this message to heart.

The trick in comparative planetology is to let the goalposts shift as you move from one planet to another. Don't look for water on Mars, but instead try to work out what chemicals can play an equivalent role under the different conditions found there. It may be concentrated sulphuric acid, it might be liquid CO2 in the subsurface, but it's highly unlikely to be clean, fresh, drinking grade water.

Huygens was very right...