All swimming pools now existing or hereafter constructed, installed, established or maintained shall be completed and continuously surrounded by a permanent, durable wall, fence or other substantial structure which shall be not less than 48 inches in height above grade and which shall be so constructed as to have no opening, mesh, hole or gap larger than 4 1/2 inches in any dimension except for doors and gates; provided, however, that if a picket-type fence is erected or maintained, the horizontal dimension of any gap or opening shall not exceed 4 1/2 inches. So-called "natural" or "living" fences or barriers consisting of live plantings shall not constitute compliance with fencing requirements of this section. No fence or wall of any kind or material shall be constructed or maintained which shall contain projections of any kind at any point on the outer surface of the fence or wall. A dwelling house or accessory in conjunction with any of the above-described enclosures shall conform to the specifications required above as to height and dimensions of openings, mesh, holes or gaps in the case of fences, and all gates and doors shall be equipped with latch-able and lockable devices for keeping the gate or door securely closed at all times when not in actual use. Gates and doors shall be locked when the pool is not in use or is unguarded or unattended.