They are water resistant, but if left out in the elements, they will leak over time.
Ammo cans are much more durable, hold more due to the "standard" shape.
Ranger Rick's site has a ammo can charcoal grill, looks like fun. You could boil water in the ammo can also.
Take a look, they might be something that you could use.
The handles are more comfortable on the ammo cans also, the wire loop on a paint can tears my hands up pretty quickly.