A drop of water functions much like the lens in a projector, spreading the laser beam into a wide cone of light and revealing the shadows of the opaque organisms swimming within the water drop.
The light rays travel from the laser in a parallel beam, but are bent together by the convex surface of the water droplet. This bending, called refraction, happens both as the light rays enter and exit the droplet. Converged by the lens-like droplet, the rays come to a sharp focal point, crossing there and then spreading out again, and continuing to spread as they travel. It is this crossing of the rays that causes your organisms to appear upside down—though it may be hard to tell since they are often quite acrobatic (see diagram below).
As you reduce the distance between the screen and the water drop, you should notice that the shadows get smaller and sharper. You can see a similar phenomenon if you use a flashlight to make a shadow of your hand near a wall. Move your hand closer to the wall, and the shadow gets smaller.
You may also notice the shadow of an individual animal change size, even when you don’t move the screen. When the animal is closer to the laser side of the water drop, its shadow will be smaller. As the animal swims toward the screen side of the water drop, its shadow will get bigger.