I am from coastal southern SC so I can give you a bit of my experience. March can be really nice or a bit cool. Be prepared. When a cold front comes through, it will be pretty windy for a few days. I am not totally sure I understand what you are looking for but given the tides and extensive marsh, it is not possible to paddle down a creek near the coast and camp. You would have to camp on an island and there are some. Day paddles near the ocean can be great and sometimes we will time our trips to go with the tide both ways. There is a nice campground near where I live, Edisto Beach State Park. Many creek paddles and the ocean, of course. There are near coastal rivers near here, especially the Edisto River where you can paddle and camp. Check out Colleton State Park in Canadys SC. I have started quite a bit upstream from there for a nice five day trip. But of course, you can start anywhere. Closer to the ocean but still fresh water is Ghivans Ferry State Park. Great for birds and photography, we fish a lot and fishing around here is well known for trout and redfish. There is also great fishing in the Edisto River.
I also have experience in the Okefenokee Swamp and the Suwanee River. It can also be nice in March, about four hours south from here. You can camp and day paddle in the swamp at Stephen Foster State Park or can start at the Park and paddle as far as you would like. At the end of the swamp is a dam that requires a short portage. Great alligators there! Nice in March because the bugs are not crazy there yet. Check water levels as that time of the year we can get some flooding.
Just a couple of ideas, you will have a great time, quite unspoiled… friendly and helpful people everywhere.
Let me know if I can help.