Have you been to the Marshwalk in Murrells Inlet?
http://www.marshwalk.com/
There's a public beach ramp next to the Belin Methodist Church where you can watch small boats going out of the Inlet to the Intracoastal Waterway. There are a lot of larger boats along the Marshwalk. Park near Capt. Dick's and walk out the Veterans' Pier for some good views.
Drive down to Marlin Quay at the end of Garden City where you can see the ocean on one side and Murrells Inlet on the other. If you get there at sunset, you can take some nice pictures from the deck of Gulf Stream Cafe, next door to the Marina.
http://marlinquay.net/
On the north end, try Harbour Gate Marina:
http://www.harbourgatemarina.com/