You try to push away from the mermaid with one final desperate surge of energy before your lungs explode. You almost make it away from her before she catches you and pulls you back down. With every fibre of your being screaming for oxygen the conscious mind that's been exercising its will to hold your breath finally loses.
You open your mouth to breathe, and your bodies cries for oxygen suddenly turn into the most intense pain you've ever experienced as salt water floods your mouth, trachea and lungs. It's as though you can feel each individual alveolus in your lungs because each one is getting burned alive by the seawater. You try to scream as you double-up in agony, your brain so overwhelmed by the onslaught of red alerts from all over your body - lungs, legs, back... the pain in your ring hand barely registers amidst all this.
You're vaguely aware that the mermaid has released her drown-hold on you. This is what mermaids did in the old stories, a part of your brain thinks with a slight sense of aloofness as your strangely-clear vision clouds. They lured sailors to their deaths by drowning. It's like having a Mr. Spock making dry observations just before the Enterprise explodes because all systems got that overloaded.
Your body tries to expel the water from your lungs, to force it back out. It's too late, of course. The damage has been done already. Still, you have to try. Exhaling is hard. Your body has gone completely into shock. You can't do it. It's like that time you fell into cold water and you could only inhale, inhale, inhale, inhale - hyperventilating without really breathing out, feeling like you were going to pop like an over-inflated balloon.
Something had to give.
And something finally does. Your chest contracts, forcing the water out! You speed away from the mermaid behind you, hovering just below the surface of the water. You take another deep breath and exhale again, propelling yourself even farther. You breathe in again, noticing as you breathe out this time that the water isn't coming out your mouth and nose like it's supposed to. Instead you feel twin jets of water flowing over your back which seem to by why you've been moving so fast.
Your eyes widen at the realization. You are breathing underwater as promised, but it's changed you. You try to slow your panicked rhythms so that you're not speeding out into uncharted waters just because you're now able to breathe and move in a similar way to a squid. Once your breathing has slowed to the point that you can tread water it's time to see what happened back there.
In the strange, rippling light of the upper ocean the first thing that you notice is that your right hand, the one that you had put the ring on, is frighteningly wrong. Instead of five fingers you now have eight. They're long. Too long. Too flexible. With too many suckers on the underside. Your right palm is nearly twice the size it used to be, and you watch in horrified fascination as you ball up your eight tentacle- fingers into the ugliest fist you've ever seen. Unclenching your hand you hold it up to your face in wide-eyed, morbid fascination, turning it over as you float there, mouth agape. Look at all those tiny suckers. There's even a small barb inside each one of them, you notice. And... the golden ring is gone. What the hell?
On the other, literal, hand - your left looks perfectly normal. That's a consolation at least... right?
It's around this time that the other strange, new sensation pouring into your brain is finally acknowledged. You're not only here, treading water slightly under the surface, but it feels like your legs are kicking in all directions at once to keep you steady. You're almost not at all surprised as you look down, affirming that the other things flitting in your peripheral vision are, indeed, tentacles. Tentacles that have replaced your legs, each long appendage swirling slightly in the water like a slow-motion whip. A slow-motion whip of muscle, sinew and rows upon rows of tiny suckers with those frightening barbs.
Eight long octopus 'legs.' Four long tentacles coming from each leg of your boxers. You look behind you to see the mermaid swimming upside down at an idle rate, though her sleek fishy body still slides through the water with good speed. "You should've seen the look on your face," she says with an innocent smile as she slides up beside you and then starts swimming playful circles around you.
You just spend a moment floating there, your brain too busy trying to make sense of all the strange sensations flooding through, including the extra legs, 'fingers' and the wildly different method of breathing, if you could even call it that.