White smoke out of exhaust

[avguy]

You wrote "water", but you really mean coolant, right? You didn't mention the model/year - any chance your drivetrain is still under warranty? Did this start happening after some maintenance or problem (coolant flush, etc)? Any overheating that you're aware of? Or, do you only use this vehicle for very short trips (i.e. no warm-up)?
I'm sure you can find folks who claim success with these sealer products, but I've always felt that if it can seal leaks, then what prevents it from also sealing other things such as radiator pathways .
Personally, I would skip the Barrs and do a compression/leak down test, and maybe a block test for combustion gas. Then, assuming the blown HG is confirmed, get if fixed because that amount of coolant loss is just going to cascade into other problems. Without an overheat, it probably just comes down to either a Monday morning special at the factory, or else just something defective with the HG - unusual these days, but it does happen.