You began to wonder and dread what "interesting" thing the mermaids would turn you two into, when you heard the mermaids begin to sing again. It was a different song than before, but still quite similar. Hearing the song gave you the strangest feeling, a sort of half-squirming in the pit of your being that made it difficult to sit still. Your breathing (you now noticed you were again breathing underwater) grew ragged once more, and from the look at your girlfriend she was the same way.
You felt hands on you, which made you shudder in anticipation. You could not see who was touching you, but you could see mermaids approaching your girlfriend, helpfully sliding off what clothing she was wearing and leaving her bare to the world. The hands on you were slipping off the uncomfortable male clothing that you had put on as well. Now, all in the water were naked, their curvaceous forms floating there with the mermaids.
You tried to swim up, tried to flee, but you just couldn't move. There was a similar thing going on with your girlfriend, who made the vaguest attempt to struggle but for the most part remained motionless. One of the mermaids swam up to your girlfriend much as she had with you, slid up her body to her, and began to kiss her, much as the first mermaid had done to you a while back. You could not take your eyes off of the two, but then your makeout partner from earlier swam in front of you.
A smile. Her hands on your cheek. A kiss.
As she kissed you passionately, tongue in your mouth, you could not help but kiss back, you mind clouded even more by the magic. You felt the changes begin to take place: your legs covering in glistening blue scales, fusing together, a fin erupting from your now-flattened feet...
When the mermaid pulled away, you and your girlfriend were now mermaids too.
"Isn't it wonderful?" the mermaid asked. "And in a few hours you will be one of us, body AND mind..." she whispered. It was true... you were having trouble remembering some things. Like your OWN name....