We used to put water in the ink cans years ago. It was fine for letterpress inks in spite of the fact that water will oxidize things very slowly. (Rusty nails in a bucket of water is your reminder.)
I'm not so sure how it would work with offset inks because water is, as my high school chemistry teacher said, "the universal solvent". However, I suspect that it would work just as well for offset inks as for letterpress.
Test it out on a can that's almost empty... that way you'll not waste much if it goes bad.
As a side note... we stopped using water when good plastic food wrap came out. We would tear off a sheet and smooth it down over the surface of the ink and up the sides of the can. Avoid bubbles. Worked just fine.
