Neither Amy nor Lina could hear Mark as they were both underwater, KISSING! This got a rise out of him, but not in anger. The sight of his girlfriend and her best friend frenching one another; it was practically number one in the boyfriend fantasy hit list. The fact that they were both mermaids on top of that was almost more than his hyperactive 18 year old libido could handle. It was then that he once again became aware of what he was holding.
Meanwhile, under the surface, Amy and Lina were busy deep-tongue kissing. Amy temporarily breaks the kiss and says, "You were right Lina; being a mermaid IS amazing!"
"You're sure Mark won't mind?" Lina said between kisses.
"Knowing him, he'd ask to join in."
The two girls shared a giggle, when they heard a splash nearby. Amy and Lina turned to see Mark in the pool; naked, fully erect and wearing the other necklace. As he thrashed about in the water, the girls swam up to him, ready to help him up to the surface. Amy stopped Lina when she noticed his facial hair vanishing, the hair on his head lengthening and jawline reshape. Clouds of bubbles escaped Mark's mouth as he groaned, and gasps took in water; but he wasn't choking. More to the point, his moans started to sound feminine.
Mark's loss of body hair and expanding and contracting chest accompanied the inflation of his breasts, as if he were inflating balloons. The narrowing of his shoulders and waist accompanied the slimming of his arms and hands; his legs, however, began undergoing a different change. They began to squeeze together from the waist down, engulfing his testicles, and leaving a noticeable gap just below his still erect penis.
By now, Mark's moans were less pained and more sensual, and Amy and Lina started becoming less concerned and more intrigued; especially now that they knew what was going on. Silvery-blue scales quickly began decorating the merging limbs and leg definition gave way to a more sleek, flexible shape. The feet began stretching and thinning, as if put through rollers; widening into a broad tailfin.
Amy and Lina swam up to the new mermaid; each putting one hand on her shoulder and one hand cupping one of her breasts.
"Welcome to the club, Mark!" Amy said.
"Or, should we say, Markie?" Lina added.
"Markie?" The new mermaid said. She immediately covered her mouth with her hands. She felt Amy and Lina's hands on her breasts. "I'm a girl!?" She looked further down to see her silvery-blue tail. "I became a merMAID!?" Her eyes trained a little higher. "But why is THAT still there?" She said in reference to the pink, fleshy shaft just above her vagina.