Hydrophobia

In a area with water (low water works fine) keep charging and shooting your gun at a malthusian and he will eventually fall down for good and drown.

Video credit: Rooster Teeth

RetroGamer87's method is correct but not always successful. I've done it where I shoot enough times to kill the Malthusian by gun.

So besides the keep shooting him down till he drowns, you can also try timing an ambush to flood water in and drown more immediately.

There are many glass doors around the ship hallways. Most rooms the doors give access to are waterlogged so to speak ;), so shooting the glass will cause a flood into the hallway.

Wait for a Malthusian to walk by and shoot the glass. Shooting him down for good measure would better the odds still. If you see an icon with bubbles highlight on the top left then you're pretty much set.

Congratulations!

NOTE: If you really want to shoot to drown, I found another way that may work better. Don't charge the gun and shoot for the legs to down him but not overly hurt him. This will keep the Malthusian alive until he becomes waterlogged himself. ;) But careful, they do still wear down and they may be smarter than you think. I've seen Malthusians retreat to a better vantage point when things got too hot for them. So just be careful and be smart. Good luck!