secret house underwater

You are someone who was swimming underwater and suddenly encountered a person who lived beneath the waves. He saw you and swam over:
"Hey, you're very beautiful. Take this stone. If you're interested, press this stone against that wall, and I'll swim over and open the door so we can hang out there. Don't be shy, come on in!"
You stood there frozen, not knowing what to do, but you took the stone as you swam back to the surface.
You were still in school and used to swim underwater during your breaks.
When the next break came, you decided to dive down to find out what would happen if you used the stone that guy had given you.
You pressed the stone, and that guy swam over a few seconds later and opened the door, inviting you inside. His home—a cave—was cozy and spacious, but for you, it was cold.