sentosa beach? you could stay the night in a hotel
would be interesting to know the water quality there in regard to the hundred of anchored container ships. but i guess it will be compared to other cities with harbors so as to not look too bad.
It doesn't look that great, quite murky. I've been in the sea over on Kusu too and it's not much better their either. However, people go diving around the Sister Islands, so it can't be that bad.
The powers that be reckon the murkiness is from the landfill going on and when I first heard this I was inclined to disbelieve them but as there is coral and plenty of other marine life around here, perhaps there is some truth in that.
Pasir Ris beach has been declared a no swim zone because of raw sewage pollution and when I read about that they mentioned testing the other beaches. Sentosa and the East Coast have been declared safe for swimming (apart from getting hit by old flip flops and planks of wood with nails in

)
But back to the topic - have you considered Rawa OP, or Tioman? Tioman is probably easier to get to if you fly, otherwise you'd have to go via Mersing and the ferry.