I revved the SeaDoo up to more than 50 mph on the Indian River in Florida, hit the brake, and held my breath. Personal watercraft, which can usually only coast to a stop after you release the throttle, have a bad reputation: Although they make up less than 10 percent of recreational boats, they account for 24 percent of accidents. Even braking could be risky, since a sudden force can send a rider over the handlebars. But SeaDoo uses a computerized system to gently coordinate an underwater brake and a reverse gear. I came to a dead stop in about 100 feet—half the distance of the brake-less rider next to me.