Is 6 hours enough time to adequately see Ephesus?
Yes.
Definitely.
If that's the time you have, it will be enough. Ephesus is very close to the town of Selcuk, which also has some interesting sites, but if you are doing this as part of a cruise, you probably will have a maximum of six hours, so stick with Ephesus. You can hire a guide at the site, or buy a carpet.