A water canon worthy of the name should be able to put a lot of hurt on the people it hits, anyway. (I got washed into a pond with a 3" fire hose once as a fireman, and there is not much you can do.)
Acid, imho, might not have any color, but nitric will turn the skin orange, iirc.
10% Hydrochloric will give an instant burning sensation on skin contact, and will do some damage if you don't get washed off.
I have had sulfuric on my skin, but only at battery acid concentrations (not sure how strong that is), and it took a layer of skin off, but was not immediately painful.
It is hard to say what other lovely things someone like Noriega might have put in the water for 'entertainment' or punishment.
As far as Amnesty or the Press, they do much better at griping where there is freedom of the press than around any real dictatorships.
Mark Twain wrote a short story fantasy about a guy meeting his 'conscience' and found out that a conscience only harangues a good guy and always leaves a dickweed alone. The story was tongue and cheek with an element of truth, but in the case of Amnesty and the Press, IMHO they've just out and out ditched their mission and sold out to the bad guys.