This government pool has two 50m outdoor pools, so it's good for lap swimming. One of the pools is shallower, and shadier too, so that tends to get the children and lessons, while the deeper pool is the one for swimming laps.
There's another, smaller pool that always seems to be reserved for classes, and a separate play area with large paddling pool and water sprays.
The only downside is that the deeper pool is next to the Smithfield market, so the end nearest the chicken stalls can get a bit whiffy!
Before you go, check if it is closed for cleaning (usually Wednesday 10-6), maintenance, or because it has been booked.