Depends on how much time you have and if you want a longer train ride. The Florence train station is only several miles from the airport (which is a nice small one). From there to Myrtle Beach will take you about 90 minutes.
If you get off in Charleston (actually north Charleston - not a particular nice area) it will take you about 2 hours and a little over 100 miles. A nice drive along the coast (but you'll have to take a short detour to actually see the ocean).
Double the distance from Savannah - a little less than 4 hours but the drive to Charleston on US 17 is great as it meanders through the Carolina low country. Both cities - Savannah and Charleston - are well worth seeing if you have the time. In fact, unless you are a golf lover, I'd rather spend time there than Myrtle Beach (so many high rise condo's). Savannah also has the option of taking the Silver Star as well as the Meteor. I'd check the internet or the local transit authority listed in the Amtrak timetable for info on the station to the airport. All 3 cities do have cab service.