Watch for Jellyfish warnings and obey if you see one posted. The last day I was in Waikiki, there was a jellyfish warning posted mid afternoon. I was watching the 6 o'clock news and 47 people were already reported stung, some in serious condition. At 7 o'clock I went out for a walk along the beach and noticed at least a hundered people still swimming/surfing along the beach. Never did hear final number of people bitten, but a sting can mean a week in hospital. Bad way to spend vacation.
Look out for jellyfish. There's something to do with a certain number of days after a full moon that makes the jellyfish come out. Find that out from the locals and stay away from the water. Waimanallo beach was really bad for it.