For sure, lost luggage and sunburn can be a bummer, but we can all agree that the best (worst) way to ruin a vacation is dying. Sadly, about one person a month is lost to a fatal accident in Hawaii while recreating. Happily, virtually all of these deaths can be avoided.
Though visitors die on the land and in the air on Hawaii, the ocean is the biggest threat to life. First thing to remember is to stay well back from breaking waves. The Islands are surrounded by 2,500 miles of open water, and each coastline is nuanced. Drop-offs at the shore are common, so stepping a few feet away from dry sand means you will be in deep water and threatened by the next big one. Stay back from the break while walking the shoreline and never turn your back on waves.
This bluff at Shipwreck Beach on Kauai invites leapers. Jumping from a high place into the ocean is a bad idea, unless the waters below have been tested thoroughly for submerged rocks. Be safe and don't jump from bluffs.
Lifeguards will post hazard signs, but most beaches don't have lifeguards. High surf is the number-one tip-off to hazardous conditions.
Even a lower surf levels will create rip current: All the water coming in as waves goes out again in the form of rip currents. You can see rip currents from the shore in the form of blue channels, like little streams going out, or as breaks in the line of breaking waves where outgoing water disrupts the surf pattern. Alway study the water before going in. Throw in a stick to see what happens to it. When in the water, float face down to see if you are moving.
Hawaiian lifeguards (watermen) are among the best, if not the best, in the world. These men and women are heroes every day. A beach with a lifeguard station is one way to increase water safety.
Head and neck injuries from bodysurfing and wave play are easy to come by. Make sure waves are breaking gently, rather than curling down in one big thump. Watch out if you see sand rising up as a wave curls to upward, as this is a sign of a shallow break.
Even when the shore is mellow, having a buddy on the land to keep an eye on the swimmer is a good idea. People get into trouble in calm conditions (cramp, jellyfish sting, heart condition).
Trailblazer guides for the Hawaiian Islands have extensive sections on beach and trail safety (and also lots of practical tips). In addition to the blanket rules, specific cautions are given for each snorkeling spot and hiking destination.
No reason to fear having fun in Hawaii, but don't treat the place like a Disneyland ride and run headlong into danger.