The following Punishment Utility Skills are blocked for underwater use.
- Serpent Siphon
- Sand Swell
- Trail of Anguish
This leaves Scourges with only a single Punishment utility skill for underwater use.
- Desiccate
There doesn't appear to be any issue with similar skills functioning underwater. Ground-target skills simply change to occurring at the target or the player location. (e.g. Well of Suffering) Leaving trails underwater also works fine. (e.g. Foul Current)
Further, there doesn't appear to be any "lore" junk that would forbid sand skills from being used underwater. (e.g. Manifest Sand Shade, Sand Cascade, Desert Shroud all work fine underwater.)
With that said, this appears to be a bug or major design oversight. Please look into it.