About how far from land is Fort Sumter?

The best answer I can find is

a 30-minute ferry ride from the Fort Sumter Visitor Education Center or Patriots Point

https://en.wikipedia.org/wiki/Fort_Sumter#Fort_Sumter_and_Fort_Moultrie_National_Historical_Park

Here is a picture from the fort, showing the shoreline. Either that is a very slow ferry, or it comes from a point in the harbour that is not near the island on which Fort Sumter is located!
https://www.postandcourier.com/news/feds-block-private-boats-drop-in-visits-to-historic-fort-sumter-in-charleston-harbor/article_6e5289c0-b856-11e9-808e-2f91ab89b8f8.html

It's about 340 metres to land from Fort Sumter.

https://www.distance.to/32.75186920260015,-79.87582373892394/32.750636990101604,-79.8791838870859

Looking at Google Maps, it all depends on where you measure. The fort doesn't completely occupy the land mass where it sits, A breakwater has been built up to help with tidal erosion, if you measure from the wall of the fort to the breakwater at its closest point it's about 20ft (6m).
https://www.google.com/maps/@32.7528076,-79.8750955,64m/data=!3m1!1e3?hl=en&entry=ttu&g_ep=EgoyMDI0MTIwMy4wIKXMDSoASAFQAw%3D%3D


From the wall to the sandbar at the estuary it's about 864ft (263m).
https://www.google.com/maps/@32.7522279,-79.8764282,185m/data=!3m1!1e3?hl=en&entry=ttu&g_ep=EgoyMDI0MTIwMy4wIKXMDSoASAFQAw%3D%3D