Thats a good question. But as much P91 as I've been involved in I do not ever remember doing hydros on it.
I would engineeer it though as requiring the water and ambient to be stabilized above the ductile to brittle transition temp which varies.
If I had a requirement for hydro I think I'd run a series of Charpies to find out exactly where it is. Though the base metal will be considerably better than most weld metal. That will be your problem.
Now, if its summer time in Nawlins, its not much of a concern, but winter in Michigan and you might want to verify.
I'd start at about +40F and verify impact strength, lateral expansion, and % of brittle fracture and then go from there. Run a few more and set up a charpy curve.