Sentosa's beaches -- which are all 100% artificial, mind you, even the sand has been imported -- are manicured and well-maintained, so marine critters are generally not a problem:
Sentosa's lifeguards prefer to be called beach patrol officers. "It's managing the beach more than just lifeguarding," said Ms Chang Shuwen, 19.
Clearing seaweed and setting traps for spiked critters like rabbitfish, catfish and stonefish are part of her job, she said.
However, I still wouldn't recommend swimming in Sentosa, since the water is filthy. Remember, Singapore is one of the world's largest container ports, and Sentosa is neatly sandwiched between the container yards of Tanjong Pagar and Jurong, with dozens and dozens of ships easily visible from the beaches themselves:
(courtesy Maksym Kozlenko on Wikimedia)
Want to take a guess at where those ships all dump their garbage, sewage, oil leaks etc? Right in those murky waters. You need to scoot across the Straits of Johor to Bintan to get even slightly cleaner water, but most Singaporeans hightail it at least to Tioman a good four hours north on the east coast of Malaysia for serious snorkeling or diving.
That said, I should note that the National Environment Agency in Singapore claims the water quality at Sentosa is "Very Good" and within WHO standards. Your mileage may vary.