A sailboat can be stopped, if detected. Thousands of sailboats are detected every day. No they can't be stopped.

In any case, why do you think it's better to give our enemies the option of using both sailboats and missiles, instead of just sailboats? I think we work on preventing the delivery systems most likely to be used against us. And the delivery systems that we have a prayer of defeating. And the delivery systems that are not financially ruinous to defeat.

It is purely a cost vs. benefits question. Runious cost for very little (if any) benefit is always a loser. It doesn't even make sense to talk about missile defense when your congress hasn't even come up with the chump change (comparitively) that would keep enemys from shipping nukes into the country in shipping containers.