Using Euro coins as weights (2004)

Water is also a convenient and accurate measure of weight if you know its volume.

I guess it depends on what kind of accuracy you're aiming for. The density/weight of water changes depending on temperature, salinity, pressure, impurities and probably other factors.

So if you're either deep into a volcano or on the top of a cold mountain and need 0.001g precision, you might want to find an alternative way :)

I think the volcano might be useful - we could use the heat to steam distill the water, then on our trip up the mountain we could take a quick stop at sea level to conduct our measurements.