Don't forget diving the rivers can be a great experience as well, Hilo- Wailuku R., Maui Seven Sacred Pools in Hana and many other places. Those areas prove to show unique indegineous life as well, such as fresh water prawn, and other fish.
Did you mean to say
swimming in the Wailuku river (and Seven Sacred Pools)?
Or scuba?
Please take the following not as a negative comment, but just as "local knowledge":
I grew up in Hilo with the Wailuku river 100 yards behind my house, and spent long hours as a teen and young adult swimming and exploring it. The Wailuku has claimed dozens of drowning victims just in my lifetime, many of them very strong swimmers. My standard advice to anyone unfamiliar with this river is heed the warning signs and
STAY OUT. There are lava tubes that drain some of the pools, and that is typically where swimmers are caught and drowned, often after being swept over low waterfalls. Access is not easy, and in most places involves walking down narrow, rocky, slippery trails. Not advised with scuba equipment.
Can you scuba dive in the larger sections? Yes. Is there anything worthwhile to "see"? Well, there are freshwater prawns. You do not need scuba to see these.
Save scuba for the ocean. Swim at Seven Sacred Pools. Look and take pictures of the Wailuku River at Rainbow Falls or "Boiling Pots". But as far as swimming and scuba: this is a river that "locals only" is a good rule to live by.