Last I knew, the DLNR specifically bans the use of crossbows when hunting game animals in Hawaii, but not for recreational use such as target shooting (where they wouldn't have any jurisdiction anyway).  If you need proof of this, simply go to any popular online seller's website that sells or stocks crossbows (even Amazon.com will do) and go through the motions of placing an order.  Sometimes Amazon will block such sales to Hawaii customers, but often not, especially if a 3rd-party seller is involved.

In any case, just visit the hawaii.gov site for the latest regs pertaining to crossbows.  There you'll find that even the crossbow ban for hunting purposes is not absolute:  there are specific exceptions made for hunters with demonstrable physical disabilities that preclude the use of recurve bows, compound bows and longbows.