A great way to teach children the value of the trial and error process! We've used this as part of our invention unit for many years with great success.
The activity encourages students to work together, brainstorm ideas, and learn from their errors. It's important during the invention process to adjust when things don't work instead of accepting defeat. By having each group build multiple boats, students are forced to adjust their "inventions" to be more successful with their second try.
While a great activity and inventions lesson on its own, it also lends itself well to be used as a full lab report as well.