At Hanauma Bay Beach Park there is a lava pool called the Toilet Bowl, when I was in Hawaii there had been a few drownings here in a short space of time. The attraction is that when you get in the pool the water surges in and flushes you out underneath....not unlike a toilet. The people had drowned when they were unable to get back out of the pool. The authorities couldn't stop people going there but advised not to go there alone. Best to watch the other idiots really!!
Every year we lose a few tourists to the power of the ocean. Listen to lifeguards. Stay back from the edge of the ocean since waves can come out of nowhere and take you with them. Always swim with a buddy.
North Shore - Winter. Need I say more? Those "No Swimming" signs are for real. Waimea Bay is usually open for swimming and boogey boarding, but use your head. If the waves look scary - that's because they are dangerous. Ladies - leave your bikinis at home and opt for the local look - t-shirt and shorts. My bottoms were around my knees after my first wave. One swimmer had his forehead gashed open during my visit. Don't swim after the lifeguards leave at sunset. If you're not confident in your abilities - don't swim at all. Google Oahu surf reports on-line and forget hitting the waves if the swells are over 10 feet unless you're an int/adv surfer. If swells are 20 feet, check out the pipeline and bonzai surf areas to watch experts catch the big ones.
The water is clear and inviting in Hawaii but watch the currents. Know you ablilities before swimming or surfing and always ask the locals about any rips or dangers. If you see no one swimming or surfing, it's probably for a reason... Don't go out.
Take notice of the warnings at the beaches. A strong undertow could easily drag you offshore and get you into difficulties.
If you are visithing the North Shore Beaches of Qahu, it would be wise to look for and read the signs posted on the Beaches.
The waves in these areas can be enormous and swimming will not be allowed.