I've literally googled, bing'd, searched every archive I know and rummaged through pages of my favorite fanfiction to find this and I'm out of luck. The only thing I can think at this stage is that this story has been deleted, which would be an utter tragedy.
It's soooo well written and the imagery is downright vivid~ I can remember whole lines of description by heart.
Basically, the Avengers get a call, and while everyone's out kicking ass Steve gets separated from the group and calls for assistance from Iron Man who tells Steve he's frying bigger fish at the moment/ stop telling me what to do/ I'm Iron Man and I do what I want. Or some variation of that.
Long story short, our dear Capsicle falls off of the docks and has a full-scale flashback, by the time he makes it out, he's soaked to the bone and freezing. Tony initially tells him to suck it up and Steve inarticulately forces out something like, "Where were you? I needed you. I needed you and you weren't there...." And it's literally fucking heartbreaking.
Tony is still being Tony and he leaves Cap to his would-be lecture, except suddenly Steve's on his knees hacking up a lung and he can't breathe.
Naturally, Tony goes from angry to concerned and is by Steve's side in one fell swoop, trying to convince him to stay awake and talk to him, but Steve is becoming less and less coherent as time passes~ eventually they make it to medical and the Avengers are all clustered around while he's hooked up to a ventilator day-dreaming/ in limbo/ dying and hears Tony's broken voice claiming that it's all his fault.
Opens his baby blues to reassure Tony that everything's alright and badda-bing-badda-boom. The end!
There is one story that I know of that is similar to this one where Steve contracts a virus from a dip in a water tank, but that's not the one I'm looking for~ Thanks to everyone in advance!!