as a temp water barrier felt is a good choice. it has been used for many many years..and those old guys knew good stuff! a 30lb felt would be best and will last for a couple of months, if your repair takes longer you may want to replace the felt. (install with button cap nails) IMO 15lb felt is not worth the $$ and shorter life span. the main issue is to ship lap so that water drains off the felt, incorrect install will add to the issues. lots of blue tarp roofs here...Katrina, Rita and most recently..Gustav. I'm a bit tired of seeing them myself! best of luck!

Update: I received the part from Amazon and replaced it myself. I placed the order at 4pm on Saturday and received it at 2pm today (Monday). Prime, 2-day guaranteed, free, via USPS. While it only had to come from Las Vegas to the Bay Area, I think that's mighty good service. I talked to Hansgrohe today, but I had no opportunity to elaborate. Their procedure was to send me an email where I filled out basic info, plus two photos at their request: the faucet and the area with the leak. Less than an hour later I received a reply that they will ship me a new part on Tuesday (which I probably won't receive till Friday, I'm guessing). It's a good thing I watched a video that explained how to replace the hose. Nothing could be simpler, right? The "instructions" that came with the part were not helpful at all, just drawings with arrows. The hardest thing I've ever had to do was to thread that hose back down through that high arc faucet. God knows what else is down there, but it took me over half an hour of up down, wiggle, curse before it emerged under the sink. It was like intubating a giraffe with a crooked neck. I don't look forward to ever having to do that again. The faucet works like a champ and I'm back in business again. I put a towel down before I started undoing things and there was a small amount of damp when I was done. I'm hoping it was just residual water left in the old hose that came out when I disconnected it. I ran the water with a dry towel there and everything seems dry. I'll keep an eye on things for a few days

It is inappropriate for someone to call the OP passive. Saying that introduces personalized, familiar, and judgemental language into a conversation with the OP who asked for advice. The person that said the OP was passive does not know the OP. If OP was passive they would not post for help. Asking for help is active, not passive.

Because it seemed important to know how bad the situation is, I removed the tiles and mudboard in the problem area. Underneath is just the sheathing. You can see that most of the wood is still in tact, but it's definitely rotted in the corner by the tub. That spot is still very wet, smells earthy, and I can dig into it with my finger. In a 4" x 4" area, I could make a little 1/2 inch hole easily. Now I'm wondering if the best solution might be to: 1) wait for wood sheathing to dry; 2) put down some kind of underlayer or mortar, and then 3) just put on some of that Flex Seal - as a temporary solution. Or, should I put down some kind of PVC board, or something like that over the sheathing - and then put mortar on that - just in case there's reason to worry about the sheathing in that corner giving in. and the PVC board would keep us from falling through the ceiling (until we do the full renovation)! Thanks everyone for your help!