Identify hentai game with mermaid in a well

[richard.freeman]

I'm looking for title of old hentai game, propably released around 2000, maybe earlier
MC visited old mansion
there was some demon theme, I think
MC found hidden entrance to underground cave, which was connected to well
there was dialog / action with mermaid there

that's all I remember

I would appreciate any help to identify that game
thanks