I personally would suggest that it is going to be a vapour and not smoke. Smoke would suggest fire/burning/ignition, which I would have thought would have had further more devastating outcome than just an escape when the fuel filler is opened. I agree that it it is likely to be a sign of water. If it is water its source will have to be traced or at sometime you are going to get serious misfiring happening, which will lead to other mechanical issues. Is there any smell of any particular note from the tank when you release the filler cap. That may give a clue if it can be identified from the fuel smell one would expect. Taking things to extremes, a sample from the tank and having it analysed may prove may be something.
Good Luck and Best Wishes, John