Yes, you can swim in the ocean at Sentosa, specifically at Siloso, Palawan, and Tanjong beaches. The water is generally calm and safe for swimming due to offshore breakwaters that protect the lagoons from strong currents and large waves. However, visitors should be aware that the water in Singapore is part of a major shipping lane, so while it is regularly monitored for quality and is safe for recreation, it may not be as "crystal clear" as a remote island paradise. Beach Patrol Officers are on duty daily from 9:00 AM to 7:00 PM, and swimmers are strictly advised to stay within the designated "Swim Here" areas marked by red and yellow flags. You should always check for "Red Flags" which indicate dangers like jellyfish, lightning warnings, or strong currents. After 7:00 PM, swimming is not recommended as there is no lifeguard supervision and visibility is poor.