Singapore has good swimming complexes by region.
All pools are open-air, and the entrance fee is low.
$2.20 on weekdays and $3.30 on weekends for adults at most of public pools.
I used to swim during night at Toa Payoh swimming complex.
As shown in the picture, public pools have 50m olympic-size pool.
It feels really great when I swim in the night, breathing with night air coupled with star gazing or when it rains so that I can swim in the raindrops.