I went to a beach in Sentosa Island in Singapore, namely Siloso beach, and I read that swimming was not allowed after 7pm on the sign pictured below. Does any beach on Sentosa Island in Singapore allow swimming after 7pm?
No, they do not. There are only three beaches on Sentosa (Tanjong, Palawan, Siloso) and they're all managed by the Sentosa Development Corporation with the same rules.
The reason you're not allowed to swim at night is because the beaches are artificial and there are strong currents right outside the line of buoys. People who go beyond them regularly get in trouble, and at night when it's dark and there are no life guards this would be a likely death sentence.