Earth and Titan prove that ocean be made of water and liquid methane, respectively. Theoretical planets with oceans of liquid ammonia and sulfuric acid are believed to exist. But what about alcohol?
There is no contradiction in a planet that has an Ethanol sea. In fact Ethanol has been detected in interstellar clouds. See One possible origin of ethanol in interstellar medium: Photochemistry of mixed CO2–C2H6 films at 11 K. A FTIR study.
Futhermore, the mentioned sea of Methane is a precedent, yet we need to consider that Methane is simpler than Ethanol. What we need to see is by what process large quantities of Ethanol would have formed...
The article linked above provides an theory for its formation in outer space. yet, we shouldn't discard the more interesting explanation for worldbuilding: Fermentation. This Ethanol sea may have formed by the metabolism of microorganisms in the planet.
It's hard to think of an inorganic process that would produce Ethanol alone in ocean size quantities - possibly Methanol if the temperature was exactly correct. It does occur in molecular clouds, but alongside methane, ammonia and water which would contaminate the whole process.
Obviously you can't have an oxygen atmosphere, so no conventional photosynthesis.
Perhaps.. if organisms evolved on a very cold planet started producing ethanol as a natural antifreeze, basing their metabolism around using H2S as a hydrogen source (so very little water - you'd need a planet much richer in carbon than oxygen), perhaps they could generate ethanol oceans.
I don't believe it will be chemically stable. To clarify I am thinking about a >50% ethanol ocean. If you look at the formula for ethanol C2H6O - that is "more" complex than e.g. water. There are many ways to rearrange these molecules.
e.g.: 2 C2H6O -> CO2 + 3 CH4
The issue with reaction to CO2 and CH4 is that they are gaseous and will "escape" the ocean, also these molecules are way more "stable" than ethanol so the reaction back will take more time and energy.
There are similar reactions to water:
e.g. C2H6O -> H2O + C2H4 (which will probably keep reacting to something else)
Anyway water and such "carbon chains" won't mix - so again back reaction will be small.
My prediction is that an ethanol ocean will over time (I suppose we are talking centuries/millenia) turn to water/methane/CO2.
Though - I suppose an ocean containing less than 1% should be quite stable.
Of course you might have a process that will replenish the ethanol. But that would be of different nature as we have here - even for yeasts more than 10% ethanol would be toxic. Also the sheer amount of mass and energy needed might be a limiting factor.