What are watertight storage solutions to protect objects from flood water?
While this borders on being a &quot;shopping recommendation&quot; question, I think a &quot;dry box&quot; or &quot;dry bag&quot; is what you're looking for.
These are often used by backpackers, rafters, kayakers, etc. to keep their items dry when out in the wilderness. Obviously, you'll need something larger than those folks will use because they usually don't take large items with them. However, once you're on the right track, you should be able to find a supplier that makes bigger ones.
You may also look at a waterproof travel or shipping case. People who take photographic or other expensive gear need sturdy, waterproof cases to protect the gear, so that's another avenue of research for you.
Finally, while I realize that a &quot;century flood&quot; can happen at any time, if you've only had your 2nd floor flood once in 26 years, it seems that moving the most critical items up to the 2nd floor is going to be your best bet when the waters start to rise. If you have an attic, reserve &quot;emergency storage space&quot; up there and you'll be above the areas you've had flooded while you've been in the house.
While it's a major hassle, your home insurance (especially if you have specified flood insurance) should replace any items destroyed in the flood, so unless the items have sentimental value and can't be replaced, I wouldn't go too overboard.
Finally, I'll reiterate my statement from the comment to be sure to have an offsite backup of any critical (or valuable/irreplaceable) computer data (like family photos/videos). This isn't the place to discuss that (hit up Super User.SE for info on that), but hard drives die, even if they haven't been swimming.