The question in my mind is why do you always see people at the last minute going out to get material to weather the storm? One would think that with the frequency of these storms the locals would have material bought at better times, on sale, and precut and stored at their abode.
By the same token what to do with your firearms collection should be thought out long before you need to put it into action.
If things happen that you don't have time to preplan, just moved in or such, the quickest and cheapest would be to use garbage bags with a plastic, waterproof, tape. Would recommend that you use 4 bags pers gun and alternate putting them over each end with taping each one separately. You mission is to create a baffle effect for water leakage. Would also recommend putting guns barrel down in safe. If gun is completely submerged no difference, but if only shallow drowning barrel will deal with water better than stocks.
What do I know? We only have Fire, Flood, and Tornadoes around here and I live on high ground.