A tidelands subdivision plat must be approved and adopted as an official tidelands subdivision plat of tide and submerged lands conveyed by the state to the borough as provided in HBC 14.14.060. The plat must show all structures and improvements located upon or partially upon the subject tidelands which were in existence on or before September 7, 1957. The plat shall serve as the basis upon which all tideland preference right disposals shall be made.