Box Jellyfish Calendar
The main danger out there in the water may not be the one you think it is. Box jellyfish, with a very nasty sting, turn up 8 to 10 days after the full moon each month; you will not want to be in the water. Check out the Box Jellyfish Calendar (www.to-hawaii.com/jellyfishcalendar.html) for when they'll be out there. They particularly affect beaches on the south shore and around to Kailua Beach on the Windward Coast.
There will be signs on the beaches when the jellyfish are around, and lifeguards have vinegar spray, which can reduce pain. If stung, get medical help in case you suffer from a severe reaction. Jellyfish stings can cause an anaphylactic shock or even death. Get the help of a lifeguard or call 911.