For people outside the USA (not USA citizens) wanting to WWOOF / volunteer in the USA / Hawaii, you will be required by immigration to obtain a 'work visa'. Work visas are not easy to obtain, but we recommend you get one.

If you say that you are coming to "work/volunteer/WWOOF on a farm, and you don't have a work visa, they WILL NOT LET YOU ENTER THE USA / HAWAII.

If you decide not to WWOOF and come to Hawaii/USA as a tourist/visitor, you will not need to have a "work visa".