I've lived in the area almost all my life. From Lakeland, you are about an hour away from St. Petersburg and a little longer (20 minutes or so more) to Tarpon Springs, Dunedin, or Clearwater. I am presently at Loggerhead St. Petersburg and am very pleased. It was nice and protected during Hurricane Irma. My other favorite marinas in the Pinellas County area are Home Port (Dunedin) and Port Tarpon (Tarpon Springs). Both give you quick access to St. Joseph's Sound to include Anclote Island, Caladesi Island, and Three Rooker Bar. I am not a fan of crusing North of Anclote Island and most of my boating takes place from Tarpon Springs to Egmont Key or north Sarasota.
I'm sure others who live in the area will provide their recommendations!