"Katie, would you still want to be with me if I were a merman instead of a mermaid?" you asked.
"Yes!" she said without hesitation. "Like I said, I'm bisexual, not a lesbian. Besides, I'm really happy you want to stay a mer instead of going back to being human."
"Good, because as much as I like you I like my penis."
Katie wrapped her arms around you and whispered sensually into your ear "I like your penis too."
Wrapping her tail around yours, Katie pressed her lips to yours and the two of you sank to the bottom of the pool. You return her embrace and untie her bikini top (which had not yet been removed) and it floats to the surface. Your lips, tongues, and hands explore each other but the best pleasure is the feeling of your tail rubbing against Katie's. You're so involved in the pleasurable sensations that you don't even notice you're breathing underwater.