Argos' inflatable "Volcano Activity Pool" can hold up to 208 litres of water - if you're not using it as a ball pit. But take note - you'll need to buy the balls separately.

Yes, it looked bigger indoors than it ever did outside. But I was committed at this point, and although my son is 10 years old and decidedly not a toddler, he was all too happy to ditch his math ...