The propane gas water heater vents the combustion exhaust vi a flute. The portable ac vents the hot air. If i only run the portable ac unit occasionally do I risk carbon monoxide poisoning from the negative pressure in the room created by the portable ac unit? I am going to die of a self inflicted wound if I don't get ac in this garage so please offer a work around for this. Should I just get a CO alarm and test the draft hood operation that way?
You definitely want a CO alarm anywhere there are people.
It is best to keep them away from appliances that can trigger false alarms.
Yes, you will get negative pressure while running the A/C, but probably not much, yet it could pull exhaust from the water heater into the room.
Portable AC units offer lots of ways to run out their vents. I'd suggest the best workaround is to place the AC unit across the room from the water heater, with yourself and the CO alarm in the middle.
Above all, think safety first.