The Twin Falls Botanical Preserve (right at mile marker 2 on Maui's Hana Highway) is where two streams with classic waterfalls, swimming pools set in caverns, and very long names join together and tumble over more waterfalls into pools—all of this lined by both manicured and wild tropical flora. Hemp kids run a sweet organic fruit stand at the trailhead. The preserve is private, with small leased farms that abut forest reserves and the whole deal is a good example of how a private landowner with vision can create great space. It's free, but why not make a donation (as prompted by a box at the trailhead) and say thanks to these people?
Don't try to make Twin Falls a first stop as part of Hana Highway day, but rather branch out from exploring around Paia. Bring your suit and camera. At the tallest, upper falls, daredevil school kids and game visitors leap from on the the rock ledges. It's easy to miss some of the falls, since the main pathway misses a couple and other pools are upstream of a confluence. Take a look at Maui Trailblazer, page 119.
Taken as a whole, and considering access and crowd issues at other pools on Maui, and other islands, Twin Falls is the leading contender for Best Freshwater Swimming in Hawaii.