[Second Edition: Have made two paragraph additions at the bottom of this post.-Ed.]
Haven't fully evaluated this idea yet, but was just thinking about the 2 vessels that have been operated by TheBoat, the M/V Rachel Marie and the M/V Melissa Ann.
Why not us them for an interisland service, with DOT either taking over their lease or buying them from the City and County of Honolulu? Many states where ferries operate successfully, the State owns the ferries, like Alaska, Washington, and Texas to name a few.
The vessels used by TheBoat actually have many of the operational characteristics that could work logistically and financially for an interisland service, unlike the Alakai and Huakai who's scale, engines and capacity were inappropriate for the distances and market. Plus, most of the Chapter 343 EIS has already been done for TheBoat, maybe only a few slight changes would be needed on that.
Some would say the M/V Rachel Marie and M/V Melissa Ann might not be able to handle the channel conditions. To that I would say, worse case they just don't operate December through February. Maybe the State runs them only intra-island during the swell months. These two passenger only vessels otherwise fit all of the demands that Neighbor island critics have raised. Plus they can make the distance in a reasonable amount of time. They would allow the harbor improvements to continue to be used. You could charge at least $40 one-way per person and I believe fill the vessel most of the time and actually breakeven for the state. With only a 6 ft. draft the State might even use the old FAP routes.
More evaluation would need to be done to determine if this would truly be realistic, but just on the main constraints that caused HSF to fail, I think this would work.
Here's a video of TheBoat. The video gets good starting about about the 3:50 minute mark. Here are some pictures of TheBoat. TheBoat is just a little small, still would work well on some of the FAP routes to quieter destinations. There are also some very interesting 600 passengers only (no vehicles) fast ferries operating in Santa Catalina, Martha's Vineyard, Boston, and in the U.S. Virgin Islands to compare to.
Hope Kallai added the following comment to this idea, "Awesome idea. Who owns them? City & County? Didn't they use Fed funds? The DEIS or whatever that prelim study was could be re-fashioned as a Programmatic EIS; then the whole project idea should really be put out to bid - with all the other ferry proposals given a chance to apply. After selection only a Supplemental (vessel-specific) EIS would need to be prepared."