Gothic 1 developers just said - "fuck it", and made a whole game based on invisible walls (well ok, they were kind of visible but in an invisible way). Problem solved.
Even then it was not just a simple wall, it didn't just stop you dead in your tracks, it dealt damage to until you decided to head back.
So they put more effort into this than Bethesda did 14 years later. xD
And in Gothic 2 there were no obvious invisible walls either. If you tried to swim off of Khorinis, you would get a warning about a sea monster and then get a cutscene of said monster approaching you and eating you.