"I kind of like that flesh tail you had. What do you say we enjoy a nice leisurely morning bath together and experiment with swimsuits later?"
"Hmmmm...not a bad idea..."
You turn on the faucet and climb in the bathtub. Amy joins you without hesitation. It's a large enough bath that the two of you can both fit in it fairly comfortably. You wrap your arms around each other and begin kissing while your hands roam over each other's bodies. After a few minutes the water level has risen enough that Amy's legs begin to tingle.
"It's starting..." she says.
You side one of your hands down her body, feeling her legs taking on a rubbery texture. Amy moans softly at your touch. You slide your other hand down to her legs as well, stroking and massaging them as much as you can reach. You can feel Amy's legs fusing together under your hands. You move your kisses down to Amy's neck as she seems lost in the pleasure of her transformation. Her hand almost absentmindedly moves to her groin. Soon you can feel Amy's transformation into a flesh-tailed mermaid is complete. You turn off the faucet and admire Amy's new form.
"You're beautiful" you whisper into her ear.
One of your hands slides up Amy' stomach and massages her breast while your lips again meet hers. Her hand slides over and massages your erect penis. You're both ready.
"My turn to be on top" she says.
Amy slides over in the tub between your legs. She puts a hand on either side of the tub to lift herself up then lower herself onto you. You wrap your legs around her tail and soon you have a rhythm going. It lasts longer than your lovemaking session the night before. Finally you both climax and collapse into each others arms.
You lay there in a loving embrace, Amy's head on your chest and her tail curled behind her back toward her head in a way legs would make impossible.