I beleive that the shooters were tangled in a net trap then drowned. If you pay attention to what is happening when they find the shooters Michaels men are fishing the bodies out of the water in the background. And i think that Fredo didn't know that it would be a hit. I beleive he only wanted them to spy on Mike. Also rember back to his conversation he has on the phone in bed. I beleive he says "You guys lied to me!" or something along those lines.
"Somebody told me when the bomb hits, everybody in a two mile radius will be instantly sublimated, but if you lay face down on the ground for some time, avoiding the residual ripples of heat, you might survive, permanently fucked up and twisted like you're always underwater refracted. But if you do go gas, there's nothing you can do if the air that was once you is mingled and mashed with the kicked up molecules of the enemy's former body. Big-kid-tested, motherf--ker approved."
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