I can't imagine what you're expecting that a dry bag couldn't handle. But if it's that bad, be sure to attach whatever container you wind up with to a completely secure object, so it doesn't float away. So you'll need a container with suitable strong attachment points, locks, cables, etc. Are you wanting to use these items you wish to protect on a semi-regular basis, or are you planning on packing them away and leaving them packed up? That would have bearing on what type of container you choose - you need to consider ease of access if that's a requirement.
Here's the plan: During the bug out, pack the car (cars if we do get a new car) with the possibility we may not be able to return home. If allowed to return to get our stuff at some later point, items in watertight containers would be there if they were not washed away, looted or damaged in some other way.
Jeanette Isabelle