You're out of luck -- Singapore is tiny, all the beaches have pretty much identical water, and that water is fugly. If it's swimming you want to do, best book a hotel with a good big swimming pool.

The nearest beach getaway is Bintan, 45 min by ferry, but this time of year the ride will be choppy, the weather rainy and the water murky there too. (And water visibility is not very good even in midsummer.)