Or does it look like this? https://www.cornerhardware.com/15inch_copper_waterheater_connector/6748_6963_7844/3726
If this is what you have I'd just replace the whole line.
Install the new one snug and turn the water back on, only tighten just enough to stop any drips. Over tighten it and your back to square one.
The rubber ring is what does the sealing, if someone over tightened it when installing it distroys the rubber seal. Most garden hose washers are not made for hot water.
Use Teflon paste pipe dope not tape on this one.