Any fic where Spike does CPR.

The closest I can think of is to walk on your knees repenting by fallrain on EF. Buffy almost drowns in a lake, Spike rescues her, and then he hits her on the back so she can cough up the water in her lungs. Not exactly CPR but I thought I'd mention it.

There is also a CPR scene in The Darkling by OffYourBird (on AO3) but just like in canon it's Xander giving Buffy CPR after The Master drowned her. Spike is watching, not because he doesn't have breath but because he hasn't learned to do CPR.

ETA: And now I found the real thing! In dreamweaver's Change Partners and Dance (to be found here, on The Spuffy Realm) Buffy drowns just like she did in Prophecy Girl and Spike finds her and successfully performs CPR.

https://spikeluver.com/SpuffyRealm/viewstory.php?sid=34536&textsize=0&chapter=5

flow

I think breath is but a soundless whisper has spike doing CPR as well. been a while since I've read it so I may be mistaken.

A breath is but a soundless whisper by Blackoberst can be found on the Spuffy fanfiction archive West of the Storm:

https://westofthestorm.com/story/a-breath-is-but-a-soundless-whisper/