Yes, it is open to public. You are only not allowed to step into the boat area of the marina. And apparently, best not to take pictures of the boats.. Security might get unhappy. Well, that was what we were told not to do anyways!
Raffles Marina is the one which the red star is on. I would not advise that you try unless you have car or motorbike, seems quite inconvenient to get there without those because it is industrial area.
Basically you head along the AYE, RIGHT before you get onto the checkpoint area, turn left into Tuas West Drive.. Don't worry if you miss it, checkpoint there still can U-turn.. But of course waste petrol la.
Travel for a while and you'll see a very prominent gate - that's Raffles Marina. Have fun shooting! =)