Hi, since this thread remains one of the first results on Google concerning this bug I'll post a decent fix I just discovered here. Download the Gothic Variablen Editor and open the saved game where the bug has occurred. Find the variables LARES_GUIDE and LARESGUIDE_ZUONAR and set each to 0. This restores the necessary dialog option when talking to Lares at he harbor. The only side effect is that he can lead you to the farm a second time. If that irks you you can set LARES_GUIDE to 1 and LARESGUIDE_ZUONAR to 2 again after he has lead you to the watermages.
Gizmo1205