The best way to travel between islands is taking flight -- only 30 min flight, but may take 6-10 hours by boat if any boat available -- I don't think there is any ferry between islands. Even you can find one, the sea-sick will kill you.
If you can plan ahead, there may be some deals for $99 (2+ people), you can have a round-trip ticket from Honolulu to any of the other three islands, plus two days rental car and one night hotel -- but the price was 10 years ago.
Have been Hawaii 4 years but didn't go back after left 10 years ago.