We also don't know what happened to the first guy, who got hit by the boat right out of the gate. But, considering that Dang was right there when it happened and she doesn't seem to have ever worried about him, he was probably at least knocked out
But we did see what happened to him: he got hit in the face by the pointy end of a boat weighing at least half a a ton, going fast enough to launch it airborne. Enough force to tear and crumple the steel pole next to him with a glancing impact, and he got hit square-on by the keel. Don't need to see the red to know his skull and ribcage are chips and salsa. If he's not dead, he's not human.
Other guy survived because the boat hit him after it hit the ground, shedding a bunch of its momentum, and hit him more belly-flop style. He still got effed up too, as we could see him bleeding through his armor from where is right arm hit the windshield frame, and he'd gone unsteady. Probably some ligament damage and a concussion at least, I'd guess.
My first thought was Dang probably did kill the second guy. She's done it before in extreme situations, even though she doesn't like it, and she really wouldn't want descriptions of her and Clint getting back to whoever commands these guys. But then I thought that unless she gets rid of that mask at the first opportunity (cool though it is), it'd give them away just as easily. So I'm guessing she cut his helmet off so she could get to the mask, and left him to the fog.
It'd be interesting if her past exposure to the fog has given her some sort of tolerance to it. We've seen in the past that people are taken over by it almost immediately upon inhalation, but she kept her mind long enough to circle around and trap that guy. Hard to tell exactly how long that was. So either she's some degree of fog-resistant now, or she can hold her breath really well under pressure.