The inlet on my water heater is leaking.
If it uses flexible connections, all you need is two adjustable wrenches and a set of supply lines. Do them both, even if the other one doesn't leak yet. Braided steel is a lot easier to work with than the corrugated copper type.

You could try plumber's epoxy putty, but you'd only be delaying an inevitable catastrophic leak.

If it's hard piped and you do call out a plumber, specify that you want it re-done with flex (so that you can DiY next time).

Some thought should be given to the inspection and replacement of your dielectric fittings (or the rectification to the complete lack thereof).