The channels between most of the islands are very rough. Any small boats including a liveaboard would result in a puke fest for most passengers.
Notice there are no ferrys!!!!
uh....actually there are a couple of ferrys operating interisland with the major "Superferry" between Oahu and Maui. (i'd post the links but i don't have that ability yet) i believe the "Superferry" tried adding service to other islands, but probably didn't get much business with the airline competition and the arrival of Go! now with the economy as it is, who knows what their future holds.
to the OP, obviously flying between islands would not really be an option unless you have a lot of time. the ferrys would give you an option to dive on two ore more islands without the altitude issue. the only major issues you may have are traveling through the HNL airport depending on your destination. the interisland terminal can be quite busy at times.
with careful planning though, you can arrive into Maui. Dive around that island, Molokini crater and visit Molokai. you can google the Molokai ferry, which provides service between Maui and Molokai. back on Maui, you can catch the Superferry to Honolulu and dive around Oahu. from there, you would have to fly to another island...Kauai or Hawaii, aka the big island, are popular choices.
the big island offers the manta ray night dive, but you may also do that on Maui. Kauai dive operations also offer trips to the island of Niihau, so if quantity is what you are seeking, that is a consideration; but be advised they only provide the Niihau trips a few months out of the year due to rough crossings. And based on your post, Kauai would be your option.
obviously, flying would require you loose a day of diving so most people just stay on one island and dive. congrats on the wedding! and have fun diving.