Have a relay race over the boats.

Put 4 boats with their bows and sterns together like a gangway.
On turn, all the crew members run across the boats as quickly as possible and jump into the water. When the first one jumps into the water, the second starts.
Time how long it takes for each crew, from the start until the last person jumps into the water.
When the first crew finishes, the next one starts.
The fastest crew is the winner.

Play the game with upside down boats.