Some prior inhabitant of Skyhold made changes to the way the hot spring surfaced -- changes designed to let the spring be used. There are carved stone seats around the far edge -- whether natural parts of the rock or not, Cullen can't say and doesn't care -- and the near edge has been smoothed down to provide for easy entry and exit, even if someone has bad knees.

That's one reason why someone might want to use them, after all. Another is that someone might have been smacked around by a former colleague who turned into a lyrium monster and is feeling a little out of sorts.

Cullen occupies one of the seats, edging down with the stone against his back so that his shoulders fall beneath the water's surface. This does not do anything to fix the appearance that he is brooding.

(He's brooding.)