I poked around "on-line" and found an 8 inch, two quart, tin Jarnagin canteen that resembles the photo. These are listed as "post" US. Civil war. I think of Cavalry units on the western plains when I see this design.

Like Reiver stated, if (in the military) you are told to carry water to thwart heat exhaustion, you will carry water. I think a belt clipped canteen wouldn't interfere with the dirk as these seem to.
I, like Richard, would guess this is a pre WW II photo.