I actually think it's probably to do with the chlorine/saline ratio; when Unley pool is more heavily patronised in summer there's more chlorine in the water and when it's quieter, proportionately more salt, which makes one float better instead of struggling not to sink.
...and how does the chlorine/saline ratio affect traffic in the area?
Correlation does not imply causality :)
But its a lot of fun coming up with implausible causalities. Try it.