Politely

"Oh, sorry, you did scare me a bit." The mermaid backs off a small amount, almost as if sitting atop the surface of the water, and simply smiles at you.

"No worries dear, I'm friendly, I promise." As if on impulse, her tail breaks the surface of the water behind her, causing a loud splash as it flicks around before falling back down. "It's kind of difficult to see, but this is just the edge of a deep series of submerged tunnels that cross through most of these catacombs. If there's somewhere you want to go, underwater is the best way to get there, provided you have gills."

The prospect does sound enticing, as it's been mostly slow going on foot this far, so you inquire further. "Are you offering something? Is there a way I could use these tunnels?"

The mermaid laughs, its clear sound echoing through the room, and you feel almost as if it reaches directly into your heart, filling it with warmth. "Of course dear! We mermaids have magic to allow air-breathers into our society, and it's use is also practical when it comes to travel. I don't know what you're seeking, but I'm sure this would be the quickest way there!"