If you have one miner left that's sane and intact enough to work, you can rebuild.
Sounds like you had a nasty tantrum spiral, That can take down the best forts. You dont know what to do, and it spun out of control. Seriously,that stuff is nasty, in my current well-developed fort, a spiral kiled 50% or 100 dwarves. Actually, it was pretty similar to you, It started flooding too, and was caused by a poorly defended fort. But with me, Winter came at that exact moment, freezing things, allowing me to drain things in a emergency move, people stopped complaining then. I rebuilt. you can Too! The dwarven motto is never say die, ever, EVER.
Check him out, is he unhappy? if so, maybe not. However, if he is relatively happy and has a pick, dig him a home, move whatever you can in there, build a bed if you can, use a burrow to get him in there, if nescasary. Injured dwarves arent useless, in fact, they're awesome. They keep going, even if they have no arms and legs, though they're a little slow. flooded forts are nigh irreparable.write off anything on the bottom floor submerged in 7/7 water. Seal yourself off, and focus on basic survival, food and beer. sounds like you got yourself a goblin problem. You dont have the time or the resources to set up even a preliminary defense. Build a wall, and wall yourself away. 1 more thing, in dwarfdom, dont give up till everyone is 1 minute from complete death. ehh, make it 45 seconds.