You can fly the aqua sheep off the screen, and therefore past the edge of the landscape, then steer it into the water, then turn it around back towards the landscape and into view. An arrow appears to indicate the aqua sheep's position off screen. Keeping an eye on the arrow it is fairly easy to manouevre the sheep even when you cannot see it.
The aqua sheep is the only thing in Worms that can enter the water without drowning.