WD40 is great stuff if you understand it's limitations. It is primarily a water displacement compound.
Its also a great penetrant, solvent, and low load lubricant.
It is NOT a good rust preventive. The film it leaves is too thin and just does not last.
If you keep spraying things with it, it will work, but once you stop, rust shows up.
Silica Gel only works if the box is sealed reasonably well. Salt air will only cause rust if it gets on the metal directly.
In other words, if you drive down the beach on a windy day. If you are more than a few miles from the beach, then your main problem is probably condensation.
The key for that is either dehumidifying, or ventilation. In other words either put your tools in the air conditioned house, or make sure they track the air temp changes.
On a cool evening, the steel tools cool down. Then, when the air warms up in the daytime, it changes faster than they do, so you have cool steel with warm humid air hitting it.
Leaving the drawers open often will do the trick, or a small fan might be necessary. You need to make sure the air temp and tool temp track pretty closely.
Also, something like this might help.
http://www.amazon.com/Flambeau-Tackle-Zerust-Squares-5x-5-Inch/dp/B002QG2ET6/ref=pd_cp_hi_3
I would look for another brand or type, since a 2 month shipping frame is not good. They are basically the same stuff you see wrapped around bearings when you buy them.
But they usually work pretty good. Since they are putting stuff out, they tend to work with poorly sealed containers like drawers.
The Silica Gel does not, since they pull moisture in from outside and its an unlimited supply.
In the meantime, there is an old clockmaker's trick you might try. Clocks must have a tiny amount of lubricant to work, but more than just a super thin film gums them up.
Rather than actually oiling the bearings and gears, they would put a small piece of cloth inside the clock soaked in an aromatic oil.
As the oil evaporates, it puts a film on the parts.
