First, the boats have to measure up.
Prior to a boat hitting the water, there are inspections to make sure they are worthy. Judges will check the weight, width, length and various other technical aspects of the boat. They are looking for anything that would give a competitor an unfair advantage over another race team. We've seen some occurrences of props that were too big or boats dramatically under weight in the past few years. In these cases, boats are not allowed on the water unless they can make the necessary changes.
Then, the drivers have to be qualified.
Not just anybody can drive these big boats. Drivers have to have put in some time in the cockpit of smaller boats, competing in at least four heats in the previous three years. Unlimited lights and 5-litres are popular places to find new Unlimited drivers.
Once they have that experience, there is a "driver qualifying" that goes on. At various race sites, new drivers will take their turn behind the wheel during time trials to build up experience in the cockpit. Before they can actually race, they must have done 15 laps in an Unlimited. Ten of those laps must be above 130 mph.
Once they've done that, they're ready to go. But as an extra safety procedure, the new driver starts at the far outside lane for the first two races - regardless of where the boat qualified in the time trials.
If you've got a qualified boat and a qualified driver, you're set to compete. Then it's on to the time trials.