This is a great bang for your buck hike. This entire hike consists of simply walking down a gravel road, culminating with a tiny scrambe down a dirt hill, with the aid of some ropes before reaching a ten to fifteen foot waterfall and a gorgeous pool (granted, it hasn't been raining to hard or not raining at all) with a deep swimming hole, rivaled possibly by only sacred falls here on Oahu. Not many people frequent this place but is very well known with locals, yet you very well may have it all to yourself. The pool is probably eight to ten feet deep at its max depth but it is perfect for some swimming and relaxation time with your friends. Many locals jump from the rocks above in to the pool, but I would not necessarily recommend this. All though not nearly as impressive, there is another little pool upstream and a few falls above that. To get to them climb up the lower falls on the left side (be careful, the falls will be slick) and then merely head upstream.
This is another one of those board of water supply hikes that I am unsure of the access restrictions. While many say it is trespassing, everyone goes to this place and again, I have never heard of anyone having any problems or being hassled while at the falls or while on the trail. I have not been back to the falls for a few months but I do know plenty of people that go all the time. There is an easement with these type of things here on Hawaii (including Maui and the emi property). While trails like these may "officially" be off limits, locals have been visiting with no questions asked for a long time. One thing that has bothered me in the past is this. As some people find out about these falls, they start going once or twice a week, probably in part because of the easy and short accessibility. I would beg that if you do go here (which I strongly recommend) please don't start going all the time. Make it a once every two months type of thing, otherwise it will become overrun like Hamama falls is beginning to be. That's just my suggestion and plea though.
To get to Naohia falls take H1East (coming from the honolulu airport) heading toward honolulu. Next take the likelike highway, exit 20A. Take a slight left on to N/Kalihi street. Take a slight right onto Nalanieha Street. Next turn left onto Kalihi street and keep driving until the road ends at the end of kalihi valley. Find a parking spot on the right side of the road. Continue on foot straight and cross a bridge and then head down the road with houses on your left and you will come to a fence. Go to the right of the fence and pass around the fence easily. Follow the beat up gravel road and eventually you will see and hear the falls below you on your right side. climb down the somewhat steep and sometimes muddy hill using some ropes and commence swimming.