Have students load small weights such as pebbles, marbles, or pennies. Determine the winner. If there is time, have the students compete to find whose boat stays upright and afloat with the most weight. Have an awards ceremony and for each category have ribbons, extra credit, or whatever is appropriate. Have the students discuss how they saw that a boat's shape determines how fast it can go and how much it can carry.
Most regattas have strict limits on the type of boats which enter.
A good example is the Americas Cup. Research the controversy around the race where the American Team sailed a Catermeran, a two-hulled boat. The race sailed the regatta into the courts and an international controversy. He offers wonderful suggestions to tie the regatta into mathematics activities.