Do you need tidal heat? What about core heat?
Ice is a fair insulator. Put enough ice on top and the heat flux from the core can keep the bottom of the oceans liquid, no tides required.
If you need the water on the surface, I don't think it's possible. Even a huge amount of interior heat (whether from tides, radiation, or primordial) wouldn't be sufficient to hold the surface liquid. The Earth possibly had periods where there was no surface water (snowball earth), and that was with the advantages of an atmosphere and solar input.
The lack of a sun is going to make having an Earth-like atmosphere difficult. In an interstellar environment, any thin atmosphere will freeze out. Without an atmospheric blanket, the surface temperature plummets.
So I think you could have an ocean, but not with surface water.
One more thing. Assuming the current heat production of the earth, and its size, I calculate that something around 5.6km of ice is sufficient insulation. Water below that depth with average heat flow would be liquid (ignoring pressure effects and only assuming temperature and heat flow issues). That's a lot of ice, but not implausible.