I don't think it is (and certainly not from Sentosa).

The equator lies south of both Batam and Bintam, which I think are the only regular ferry routes from Singapore.

I tried to do something similar 20 years back but got fed up with the weather. I was going to have to traverse both islands then get a ferry on from there to Sumatra in my quest to cross the equator.

I realised a few years later it was a lot easier just to fly to Australia in business class....