Can I use duct tape instead of a draft hood?
Unless your hot water heater is positively vented (i.e. an exhaust blower), you need a draft hood in order to prevent back pressure in the vent line.  If you connect directly to the hot water heater without a draft hood, you are risking having the pilot light blown out by pressure in the vent (think wind) or letting exhaust gases escape into your house. It's a bit fuzzy in your picture, but it appears that the heater has attachment points for a draft hood. If that is correct, don't omit it in your venting.