Box jellyfish These turn up eight to 10 days after the full moon of each month. Follow beach signage and talk to lifeguards. See www.to-hawaii.com/jellyfishcalendar.html.

Car break-ins Absolutely do not leave anything visible in a rental car. Car break-ins are common. Hiding things in the trunk is only effective if you do so before getting to your parking spot.

Beaches Do not leave valuables on the beach while you swim; slippers and towels are usually left alone.

Swimming at waterfalls Hazards include falling rocks and leptospirosis (an infection caused by corkscrew-shaped bacteria).

Theft & Violence

Oʻahu is notorious for thefts from parked cars, both locals’ and tourist rentals. Thieves can pop open a trunk or pull out a door-lock assembly in seconds. They strike not only at remote trailheads when you’ve gone for a hike but also in crowded beach parking lots where you’d expect safety in numbers.

Try not to leave anything of value in your car anytime you walk away from it. If you must, pack things well out of sight before pulling up to park; thieves watch and wait to see what you put in your trunk. Some locals always leave their cars unlocked with the windows rolled down to avoid paying for broken windows.

Stay attuned to the vibe on any beaches at night, even in Waikiki where police patrol, and in places like campgrounds and roadside county parks where drunks, drug users and gang members hang out. In rural areas, there may be pockets of resentment against tourists, particularly on the Waiʻanae Coast, where homeless encampments have taken over a few beaches.

Flash Floods & Waterfalls

No matter how dry a stream-bed looks, or how sunny the sky above is, a sudden rainstorm miles away can cause a flash flood in minutes, sending down a huge surge of debris-filled water that sweeps away everything in its path. Always check the weather report before starting a hike; this is crucial if you’re planning on hiking in valleys or swimming in natural pools or waterfalls. Swimming underneath waterfalls is always risky due to the danger of falling rocks.

Tell-tale signs of an impending flash flood include sudden changes in water clarity (eg it becomes muddy), rising water levels and/or floating debris, and a rush of wind, the sound of thunder or a low, rumbling roar. If you notice any of these signs, immediately get to higher ground (even a few feet could save your life). Don’t run downstream – you can’t beat a flash flood!

Tsunami

On average, tsunamis (incorrectly called tidal waves – the Japanese term tsunami means ‘harbor wave’) occur only about once a decade in Hawaii, but they have killed more people statewide than all other natural disasters combined. The tsunami warning system is tested on the first working day of every month at 11:45am for less than one minute, using the yellow speakers mounted on telephone poles around the island. If you hear a tsunami warning siren at any other time, head for higher ground immediately; telephone books have maps of evacuation zones. Turn on the radio or TV for news bulletins. For more information, visit the Pacific Disaster Center (www.pdc.org) and Hawaii State Civil Defense (www.scd.hawaii.gov) online.

More Information

In general, Hawaii is a safe place to visit. Because tourism is so important, state officials have established the Visitor Aloha Society of Hawaii (https://visitoralohasocietyofhawaii.org) which provides non-monetary emergency aid to short-stay visitors who become the victims of accidents or crimes.