I've got a couple of 35mm film canisters out there, and they're certainly not watertight - they need to be in pretty sheltered spots to remain dry inside.
My own favourites are lock 'n' lock boxes - they have really good seals and provided people close them properly they're absolutely watertight. I do realize that the same could be said of ammo cans, but personally I don't like those, as they're noisy to open and close, and so can be a dead giveaway in quiet spots. They can also be a pain to open sometimes!
No, I don't think using one's coat or whatever to keep a cache dry is particularly successful - it may (or may not) keep the rain out, but in heavy rain the air's laden with moisture which is going to get shut in the cache when it's closed and then condense.
Do not go gentle into that good night.
Rage, rage against the dying of the light. (Dylan Thomas)