Water for flushing could be more important than you know,,,,sanitation wise. If the power is on-off-on-off for any length of time, the water in your tub is critical. We also know tub stoppers work about as well as a sieve.

I read a site a while back that said spray some liquid foam into your drain orofice, and the tub will hold water. My problem with that is that this foam stuff really can be intrusive in your drain lines, is hard and not maleable at all, and hard as heck to pick out when normalcy returns.

So here's the remedy. Any piece of plastic,,,a Heft trash bag or a Ziplock sandwich bag. Cut to fit the drain enuff to come up the sides and then some, then cram a glob of Playdough into the drain. It makes a water tight seal, and when you want to dump the tub, pull on the edges of plastic. Re-useable over and over and over again, and no picking foam out of everywhere.

Good Luck