I'd be surprised if you're able to receive Marion or Darlington Palmetto 800 sites from the Myrtle Beach area, unless you're staying more toward Conway than on the coast.
But there are three Palmetto 800 sites that you probably can receive: Site 61 North Myrtle Beach, Site 39 Georgetown (which has a simulcast site in Pawleys Island), and Site 38 Conway. You'll typically hear the same traffic on all three sites, though you'll hear the Georgetown Co. talkgroups (SD, PD, FD/EMS) on the Georgetown site.
Also, in addition to the Horry Co. and MB TRSs, don't forget about the North Myrtle Beach TRS. And if you're into utility monitoring, there's the SC Public Service Authority TRS (that's Santee Cooper) active in the area.
Also, there are several trunked Business SMRs (Myrtle Beach Communications and Waccamaw Wireless) active in the area too.
Most or all of these systems should be listed in the RR database.
I'm not really sure what you mean by APB communications systems. If you mean for Amber Alerts and the like, you'll hear those broadcast on most of the LE dispatch channels (Horry Co. PD and SCHP).
Hope that helps.
Brian