You stretch, but the angle you have fallen in keeps you from reaching the handle. What would you do if it opened anyway? Say a costume turned you into a merman? Well, you could cross that bridge when you came to it...if you came to it, considering you were now gasping for air.
Suddenly, the floor disappears below you, and you experience that nanosecond of weightlessness before the fall. It was a trapdoor! But where does it lead? Before you can reach for something to stop your fall or make a sound, you tumble into the darkness. With a splash, you hit the water below and begin to sink.
Panicking, you hold your breath as long as possible, but as you drop, you finally give up and let the water enter your mouth. You prepare to drown...but immediately feel much better as the water flows over your gills. GILLS?
Reaching up, you feel the slits along the side of your neck pushing water through, and strangely, it feels natural. Your eyes are slowly adjusting, and despite the lack of light, you can make out the sides of a vast underwater tunnel or cavern. The water is flowing gently in one direction, and you have already been pushed a ways from the trap door. Moving yourself about, you find you can swim unexpectedly smoothly thanks to your new lower body. It even gives you quite a bit of speed underwater. You might even enjoy this if it hadn't torn you apart from everything you know.
Getting more comfortable in your new skin...and scales, you try to determine the best course of action. After considering everything, you decide to...