Hey,
I'm in zone 7 too. I have several leaves that I made more than 3 years ago that have been outside all the time. They catch rainwater, and I haven't noticed any cracking. a couple are unpainted and not sealed...I keep meaning to paint, but just haven't. A couple are painted and sealed, I can't really tell any difference.

And yes, you should change the water when it gets funky in the water bath.

I have birdbaths sitting outside for about four years in zone eight with some temps in the teens. No problems. I used no waterproofing, just a routine mix coated on the inside with a slurry of one part cement and three parts sand. My philosophy is--If my hypertufa cracks, I will make a new one.

I tried the "Zone" link and it didn't work, I'm in WI and we get sub 0 F days and nights. I just learned about hypertufa. I THINK I would like to have this on the bottom of my waterfall which is 2' wide and 12' long. I would like to lay real rocks (about fist size)on it. My biggest concern would be will I have to put driveway sealer on this to keep it from seeping/leaking?
Also, I don't want to use concrete because I'm afraid it would crack. Is hypertufa less prone to cracking, espceially in winter? Thanks

I am in Wisconsin zone 4 (ridiculously cold!!) and I leave my hypertufa out all winter. Some I cover and some I leave and I have had only 1 crack... but it was a pretty thin piece.