The question is pointless without something to compare water to.
Technically it is exactly medium because most people use it as the standard for measuring density.
Since it cannot be compressed - (only pressurized) I would have to say a high density. This is also why Hydrolics are so powerful.
It's relatively high compared to something with a lower density... such as air.... yet, it's density is relatively low compared to something with a higher density... like iron.
It really depends on the temperature, Water vapor is low, ice is high, liquid in between.
In comparison to what?
Are you designing an atom bomb?
High - but in comparison to what?