Just a heads-up so others find out about this before it's too late.
Early in the game you will meet Lares (an old friend from the first game). When you ask about the mercenaries he will offer to take you there. In the original Gothic II, that's fine; with Night of the Raven, however, when you next talk to him the "let's go" option will be missing, which means he is unable to join you on two of the add-on quests he is intended to help you with. If you also give him the quest item you are told to take to him you will be unable to get it back from him and finish the associated quest without cheating.
If you have already done one or both of these things, the best thing is probably to just start over if you haven't gone very far into the game. Even if you cheat to get back the quest item (if you gave it to him) you still won't be able to get Lares' help on these quests, one of which will be quite difficult to do alone until you have built up your character.

I suppose if you cheat to get the item back you might as well cheat and finish the damn quest all together. If I understand correctly which quest you mean it involves nothing more than walking from A to B and giving that item to an NPC.
The problem with him not having a dialogue option for helping with the quest did happen to me as well, but by that time I was high enough level to finish it on my own. It is annoying though.

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.

Anyway, the advantage of this method over using the cheatmode to place the ornament in your inventory (which seems to be the most popular solution on the web) is obviously that Lares will lead you to the watermages and basically things work as intended. Preferably I would have just resorted to one of the community patches but the earliest ones use an installer that's incompatible with the GOG version and the later ones require you to start a new game due to compatibility issues with the original version's save files.