Hello, fellow open water swimmer here.
Let's say you're not looking into joining a team locally, Singapore is pretty bad when it comes to options to swim. Max Headroom is right about East Coast Park, the stretch between Bedok Jetty and NSC. In fact that's the most common stretch used by aquathlon, triathlon or swimathon events. Another stretch would be Sentosa island on Tanjong Beach side (pretty limited), and Changi Coast. Visibility, water quality and risk of accident is not great if you compare with other countries.
I always, always prefer to swim in Malaysia or Bintan, where they have better water.
For local teams to train with, join Meta Sports https://www.metasport.com/
. It is a triathlon training club but it has weekly open water sessions at Tanjong Beach. I have not joined any other group.