Hammacher Schlemmer offers a 2 feet long remote controlled Shark.

I am not sure if you can scare people with this shark circling around in a pool.
The 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.

Hammacher sells the robot shark for $99.95.
The Ugobe PLEO will top the robot shark in life like movements this coming holiday season.