How to Fix this Water Damage?
This type of damage in that area adjacent to a tub enclosure is common, and I am going to suggest that this area has seen prior repair attempts.

I also wonder (due to the vertical seam in plaster board) whether you would find wood members for adequate support and fastening of new plasterboard, if you were to tear out and replace all the existing questionable plaster/plasterboard.

If the remaining board is structurally sound (dry and firmly attached) your easiest play here might be to use a hard setting plaster (like fixall or water-putty) to cover over the area. Hard-setting plaster is tough to get smooth and practically impossible to sand, but is resistant to future moisture damage due to its hardness. I might patch with the hard-setting plaster but not to wall level, leaving room for a skim coat of regular plaster compound to allow sanding to a smooth finish.