I'm also in the middle of preparing for our Hawaiian honeymoon cruise. While I haven't booked anything through it yet, I did stumble upon the following website:
It seems to have quite a few local excurisions. For most of the major islands too. I've compared the prices to carnival and it's much cheeper. If you book directly (not through carnival), make sure that you have some way to get to the excursion. Many will come and pick up everyone from a boat. Just makes things a lot easier. Like I said, I haven't book through the site so I can not speak to th quality of the excursions, but I'll bee looking into it again pretty soon. Best of luck!