Taking a quick look at their website, it appears as though the quality of diving will be very much dependent on which itinerary you book.
I would think you would want to book the Cultural or Heritage options if your main concern is the diving. Either of those would provide world class diving.
If Fijidiver sees this thread and responds, you will get a much more authoritative evaluation of the relative merits of the Cultural and Heritage routes. In case he does not....
The Heritage cruise looks like it spends more time in the heavily trafficked Tavenui and Savusavu areas. One of the reasons they are heavily trafficked is that they present fantastic diving, including the Great White Wall and Namena Marine Preserve (you can easily find information on these areas on google). This route also makes a stop in the Laveuka and Ovalau area....hopefully to dive the Wakaya Passage where you have a good chance of seeing great hammerheads and manta rays. Wakaya is fantastic diving and is only frequented by a couple of liveaboards, Wakaya resort (VERY Pricey), and Naigani Island resort (small family friendly resort that has only started really emphasizing their diving this past year or so. They recently were given an Award of Excellence by Trip Advisor).
The Cultural route appears to cover more ocean. It also passes through the Tavenui and Savusavu areas but does not spend as much time there, and no stop in the area where you could dive Wakaya passage either. On the other hand, this route takes you to some places that are a bit harder to get to and so they see far fewer divers. It looks to me like the highlight of this route may well be Yadua Island. I havent been lucky enough to dive there (with a non diving wife liveaboards are not something I have done yet) but I have heard really great things about the diving there....I cant wait to get a chance to dive it in in the future.
If you decide what route you will be taking, or have any more specific questions, feel free to ask. If you get lucky fijidiver will stop in and get you much more detailed info (especially on Yadua, which he knows well).