How can I attach things to the inside of a shipping container without compromising the water tightness?
You can do this without disturbing the walls or ceiling of the container.  Frame thin wall sections on opposite sides (use 2x3s instead of 2x4s to minimize the loss of width).  Run 2x3 joists between the two sides at the ceiling.  You will then have a free-standing, three-sided structure.  

With it tight against the walls and ceiling, it can't go anywhere, so you don't need to worry about bracing it so that it doesn't tilt.

You can mount to it, hang lighting fixtures from it, etc., without having to secure it to the container.