Beginning part of the fic has Steve and Tony walking down some dark cavern, maybe looking for Hydra related villians? There's water at some point and Tony starts having PTSD issues, and perhaps his story comes out to Steve? Steve gets protective, possibly even promises himself that he's going to protect Tony now. This may be part of some series?
Ugh, is that enough for someone to help me out with? Lol. I hope so! :)
** Found! **
Missbecky's "The Mission" series (the first installment)