A somewhat random addition to the post below. Norman Boyd recently sent me over the above scan of a Stingray advert drawn by Ron Embleton from July 1965. For you youngsters, 15/6 was fifteen shillings and sixpence, or 77 1/2 pence in "new money". That's pretty cheap as the retail price index is probably only 14 times greater today in comparison to 1965, making the cost of the inflatable around £10.75 today if we were only to consider average price increases.
You can see what the inflatable looks like here.