Waikoa Pond (or Venus Pool) is at mile marker 48 on the Hana highway in Maui. You have to break the rules a little bit and go past a barb wire fence, follow a trail, past an old stone oven(?), down a bit, and then you're there.
Apparently in old Hawaii, this swimming hole was reserved for royalty. You can totally see why. It's pretty cool and there were only a handful of people with us hanging out, swimming, and diving. The pool is at least 15 feet deep and it's very calm. There's some fish swimming about as well. Beyond the pool is the ocean full of grey sharks and fairly rough water.
I'm glad I brought my MacBook and that we had really good Web access over in Hana, otherwise, I never would've found this place. It'll probably be the highlight of our trip, it's a pretty neat watering hole — the water is calm and swimmable, there's a black sand beach on the other end, lots of rocks to dive off of, and you're just surrounded by cool scenery of bright green against black lava rocks. The best parts though are there's not that many people around, certainly no one in the water, and few will ever find this place.
Did I say mile marker 48? I meant mile marker 38. Yeah, that's it.