Kauai Nude Beaches
I live on Kauai, and have for 4 years. I go to Larsen's on a 2x/week basis, and have only seen police one time: Labor Day, when a local busybody made a fuss. All the cops did that day was ask us to put something on. No tickets, no arrests. And other than that one time, have not had any problems at Larsen's. There are quite a few local regulars that go there.
Donkey isn't closed (don't know where you heard that), but nobody goes nude there anymore because of the trail the county installed nearby that overlooks the beach. Its too visible for comfort.
I can't speak for Secret Beach firsthand, but I've heard second-hand that it is still ok. There used to be an issue with the landowner above the beach, but I don't know if they still grumble about nudists or not.
Legally, the issue of nude beaches is grey in Hawaii. There are no technically legal nude beaches in the state, but the statute regarding indecent exposure is worded that if someone is nude around other people who are also nude, it's not illegal because there's no chance of being "affronted or alarmed." (HI v Kalama)