Portable AC in garage. Concerned about safety issue.

I had an AC guy suggest a portable AC unit for my garage. It's a fairly large unit and I had to cut a whole for the exhaust (not sure if I'm using the right term). It's a two car garage.

My concern is the fact that I have a gas water heater in the garage and I'm afraid the unit may be sucking the water heater exhaust out of the flue and into the garage.

When I turn the unit on my garage door gets sucked in a bit. Also when I open the door to the house you can feel it being pulled shut and you can hear air being sucked in through the opening in the door.

The guy told me that I shouldn't be concerned because any carbon monoxide that gets pulled out of the flue would quickly be sucked out of the garage by the AC unit. Since it seems to be creating suction it seems like the AC unit is pulling more air out than it's putting back in causing negative pressure.

I don't want my family to be on the news because their idiot dad set up a dangerous situation in the garage leading to carbon monoxide build up that seeped into the house and killed them all. Is this a concern or no big deal?

Portable AC in garage. Concerned about safety issue.