I usually fill one or more of our tubs as a storm approaches, just for washing and flushing. I wouldn't be afraid of drinking it though if we had to. Could always boil, filter or treat it.

I use a little clear packing tape around the drain to seal it. That cleans up a lot easier than RTV or other sealant. Or lining the tub with plastic sheeting or a plastic shower curtain will seal it in a pinch.

45 gallons of water is not that expensive to store. I store a lot of 1 gallon jugs from Wal-Mart at about 60 cents a gallon, including container and packed 3 jugs per box. 2 liter soda bottles are a lot sturdier and are essentially free.