Up to you. Sneaking them in if she feels like it, refusing if not in the mood. I can give you a preview of the water near next mingle's location, or you can make up an environment of your own, like a tropical island or something.

Any two bodies of water deep enough to swim in. Best results when she and the passenger are strong swimmers.

Price is up to you. Each Fae has individual needs and desires, even if she's purely a siren and not a Fae--though I think lore would change the dairy popular with Fae to something else.