I have kept a boat at St. Marks in the past and the Fort is not much but the coastal drive is great. We have been to PC too many times to count. While you are there try Uncle Ernie"s in Old St. Andrews it is on the bay (best view of any waterfront eatery) and very, very good. (try the Ensalada salad) If you like a little drive try Port St Joe and Joe Mamas Pizza (try the Quatro) on the main drag. Top 100 on one survey a few years back. We live 35 miles from PSJ and 65 miles from Uncle Ernie's and do each 1-2x/month. I would not make St. Marks a destination. Go a little further 20 mi) from PSJ to Apalachicola and you can spend a great day in a old fishing village shopping and eating (the Owl or Up the Creek) 9 more miles puts you on St. George Island...real real nice only one eatery right on the gulf but the view is worth it. Or, if you are on the Island from 3-4 try Edie Teach's raw bar for 1/2 price oysters...lots of colorful locals and the best oysters you'll find.
If you go west of PC try Cafe 30A in Seagrove Beach..5-6 every day 1/2 price entires and we're talking fine food!
Enjoy