My current truck has a Trail FX box in the bed which has a rubber seal around the lid. I've had it through car washes and no leaks. My previous truck which I owned for 12 years had the same type of box and never had any moisture in it. I carried many tools in it and never had any rust on them.
Now I will say I live in the southwest where humidity is low, but that truck had been through torrential rainstorms, hail, snow and assorted bad weather and hundreds of car washes and never had any moisture leak in.