Hi, I’m doing the Mermaid quest and I have all the ingredients needed to make the Fishman potion. I keep going to the witch, but she won’t take the money or make me a potion. She says two dialog boxes, and then a stand-alone dialog box appears. I don’t see any potion in my inventory and I can’t get into the water. I don’t understand kana, so I don’t know what she wants. If anyone needs me to, I can post screenshots of what happens. Thank you very much.
Do you have enough money? It takes 10 000 crowns IIRC. Anyway, you should check WeekendRisktaker’s RS2 faq, it will be way more helpful than me 
Link:
http://db.gamefaqs.com/console/snes/file/romancing_saga_2_d.txt
Yeah, I have 10,000 gold and did everything he (Weekendrisktaker) said. I still can’t get the potion, though, and the old woman keeps telling me to do something.
Be sure you finished getting the Water rather than keeping the Moonlight Comb and trying to buy the potion with it.