"75% of the damage is caused by what we are doing above sea level."
And 72% of all statistics are made up on the spot. Our actions on land have varying affects in the ocean depending on the area. Global warming is an issue that could have all kinds of effects ranging from simple adaptations in most of the world's organisms to mass extinction. Unfortunately, according to Sepkoski (1997), over geologic history, species have gone extinct at an average rate of 2 to 3 per year. In the past 200 years, they have been averaging 74 species per day, and we have yet to see the effects of a warming earth on global biodiversity. With regards to Hawaiian marine biodiversity, the big concern is coral bleaching which has happened, but on a very limited scale. Hawaii is far enough from the equator that most of our corals aren't living at the threshold of bleaching like those on the GBR.
Ocean acidification is going to be a problem for any calcium producing organisms. There are three kinds of biologically available calcium, and some fare better than others. One study subjected temperate pteropods (shell producing organisms) to the projected ocean pH of 2100 (Orr et al, 2005). Instead of growing, their shells actually started to dissolve. This isn't a a huge problem now, but will be and as 3000 psi said, it requires immediate action.
However in Hawaii, while eutrophication, sedimentation, heated effluents, and ship groundings have had localized effects, the biggest problem is the sheer amount of critical players in the food web that have been removed. Some species play some VERY important roles, but the fishing pressure isn't directed at those species whose role is minimal, but instead at what tastes good. We have eaten our way through the apex predators that keep other species in check, and are now working on the grazers that keep algae from taking over our reefs. Had Kaneohe Bay had its grazers in place to take care of the native Dictosphaeria (Gracillaria and Kappaphycus came later), it probably would have fared much better in the face of the sewage outfalls. Given, the Northwestern Hawaiian Islands have a slightly different species assemblage than the MHI because they are slightly cooler, but the community structure and biodiversity are much different because they don't have local fishing pressures. I used to be a rabid fisherman on the mainland but have stopped after seeing example after example of what it has done to the reefs here.