not an hvac guy, but a tradesman with experience. I cannot see the vent hood in that photo so I cannot speak for its installation. It appears that the elbow is very lightweight dryer vent, would prefer a 26 gauge elbow that can handle the heat and be generally more rigid. The connection is pretty loose, which could both effect draft and discharge. Tighter fit with mechanical fasteners, tradesmen probably debate the use of foil tape or not. I cannot say. It doesn't need to be dead on airtight, but certainly better than what I see. What concerns me is did the bypass a vent hood and is that sloppy connection acting as a vent hood. The ins and outs of venting an appliance are not difficult to understand, but they can be critical. Also make sure you have a CO detector near there (probably required in your sales agreement). [Here](https://inspectapedia.com/plumbing/Gas-Water-Heater-Venting-Codes.php is a little primer.