e g between 214 and 215 it may be possible to create a watertight joint on assembly and thereby dispense with an inner waterproofing liner the watertight liner 304 may for example be a shaped sheet of a plastics material such as polyvinyl chloride permeating and surrounding a woven structure of polypropylene or similar fibrous material preferably the liner is durable wear resistant and resistant to the action of any additives put in the water for bactericidal purposes or the like and preferably it is resistant to ultraviolet light for those instances where the apparatus is used outside we imagine that the invention will usually be used in a garage or a basement room preferably the liner forms a basin of approximately the same dimensions as the cavity formed by the assembled panels within the frame 100 and preferably the open border of the liner is provided with a number of fixing points 307 so that it does not sag down and allow the water to escape one preferred attachment means is shown in fig3 where the liner 304 emerges from the depths of the pool to cover the upper support beam 302 and is passed under one of a number of fixing studs 307 which may be a press fit through apertures into the beam another less preferred means for fixing.