I visited Ahalanui Hot Ponds in September 2013, when I was vacationing on the Big Island of Hawaii. The drive from Hilo was stunning, and Ahalanui Park was gorgeous. As you can see from one of the photos, there are warnings about bacteria in the water. When I was preparing for the trip I searched the internet for information about the hot springs and found varying information about the cleanliness of the water:

http://www.lovebigisland.com/hot-ponds/

htttp://www.discoverhawaiitours.com/travel-guide/ahalanui-park.html

http://www.yelp.ca/biz/ahalanui-park-aka-pualaa-county-park-pahoa

http://www.tripadvisor.ca/Attraction_Review-g60603-d560109-Reviews-Ahalanui_Park-Pahoa_Island_of_Hawaii_Hawaii.html

There certainly were people swimming in the hot (warm) pool when I arrived. However, I decided not to go in and just dangled my feet in the water. It was lovely.