It is not free, you need to pay for the boat ride, but fees aren't too bad.
Yes and the boat ride is quite relaxing too. We didn't visit the aquarium next door but it looked interesting too.
You can also go over to Patriots Point and go on a aircraft carrier and inside a submarine. Our kids love going on those. It's not free either but worth every penny. You can also catch the ferry to Fort Sumter fromthere.
It does cost some money for the ferry, but worth it. I think the boys would really enjoy both the ride and the fort
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