According to an MIT news release, several researchers including John J. Leonard have been hard at work on a scheme to create an ultra-cheap fleet of sea-going robots made from - $500 plastic kayaks. To avoid the problem of communicating underwater, the researchers decided to stick with a surface robot and off-the-shelf kayaks proved to be an inexpensive solution. The kayaks, dubbed SCOUTs (Surface Crafts for Oceanographic and Undersea Testing), are packed with computers, propulsion and steering systems, and radio communication gear. 10 SCOUTS have been built so far and more are on the way. For more details and photos see the paper, SCOUT - A Low Cost Autonomous Surface Platform for Research in Cooperative Autonomy (PDF format). For more on the autonomous navigation system used by the SCOUTs see the MIT COLREGS-Based Naviation of Unmanned Marine Vehicles website.