The first time I saw one was when I was a kid at the Philadelphia Zoo. It glided across the water as elegantly as a real swan. I saw the swan boats and I knew they were something special.
What makes the swan boats so great isn't that they look like swans, but rather that they are boats that move via pedal power. Just imagine! A boat you don't need to row with unwieldy paddles. This leaves your hands and arms free for holding important objects or for interacting with things. I love biking, and boats, and this.
The only downside to the swan boat is that it often requires another person. Nobody I know wants to be out on the boat as long as I do, so going it alone in a pedal kayak is perfect.
The pedal kayak sheds the unnecessary bulk of the swan boat, and brings a sleeker, more refined design to the table. It is clearly a kayak, and I suppose you could paddle AND pedal...