Fort Clinch area fishing and access to Cumberland Island

There is no boat launch at the park, but there are several marinas in the area. The park site has some info on fishing: Fort Clinch Fishing

The sites on either campground at the park are fairly small, so you may need to park your boat trailer elsewhere. The beach loop has some space available over by the tent camping area, but I don't know about the river loop. Also be aware that there are strong currents and a busy boat channel where the St Mary's River enters the ocean.

You may find this information helpful about accessing Cumberland Island by private boat:
Can a private boat dock at Plum Orchard on Cumberland Island?