Robotic Shark
It’s two feet long, swims more or less like a real shark. I am not sure if you can scare people with this robot shark circling around in a pool. You can try it.
This robot shark is able to maneuver up, down, left, right and backwards through water, in depths up to 9 feet. The shark can swim up to 40 feet from its handheld remote unit which is also submersible, allowing you to swim with your shark. The battery only holds for a 15 min swim.
Price: $99.95
[Via Hammacher]