Bring a hat, swimming costume and light clothing that you don't mind getting wet. You'll need to take your shoes off when you get on the yacht so you should bring flip flops, or something easy to take off. Do bring lots of sunblock, and a change of clothing just in case your clothes get wet!
Many people also ask if you need to bring your passport along for a yacht day out - you'll be in Singapore waters so you don't need to!