Every person who shall own or be in possession of any premises on which there is situated on the effective date of this chapter, or at any time thereafter, a swimming pool, which has a water depth of 18 inches or more in any portion thereof, shall maintain on the lot or premises upon which such swimming pool is situated a fence, wall or other adequate structure completely surrounding such pool to make the same inaccessible to small children. Such fence, wall or other structure must be not less than four and one-half feet in height with no openings large enough to admit a child except through doors or gates as hereinafter provided.