This is the 2nd time I've made multiple dives w/ Seasport Divers in Kauai, and I continue to be quite happy with them a. They run a very nice operation - friendly, helpful, very knowledgeable about all prime dive sites, aquatic life, and have great boats, boat captains, instructors / DM's, solid rental gear (if needed), etc. Slightly more expensive than some of the other dive outfits, but definitely worth the small (like $10 - 20) "premium".
Besides Ni'ihau which is a must do 3 tank trip from Kauai, the south shore has several great dive sites w/ lots of fish (relative to Oahu, and my home turf of Puget Sound), sea turtles, octopus, monk seals, sharks, etc. On the north shore - tunnels beach continues to be an awesome shore dive spot - you just need to be watch ocean conditions which can drive some strong currents.
On Oahu, I tried to do more wreck diving and went out with 2 different outfits - Aarons Dive Shop, and Ocean Concepts. Aarons was [relatively] inexpensive and provided Nitrox at no additional charge (really nice!). Their boats were relatively small (8 divers max - no walking around) and didn't have much in the way of amenities, however the dive sites are pretty close so, having a small group isn't necessarily a bad thing at all. Ocean Concepts was the other shop I dove with. They had a much larger boat, and was really thorough on safety, gear checks, pre-dive briefings, etc. They were slightly more expensive ($10 - however even on a morning trip, they provided lunch) than Aarons and charged $10/tank extra for nitrox. All in all, I was really impressed w/ Ocean Concepts but was almost equally as happy w/ Aarons (esp. as I primarily dive Nitrox) given the total $'s difference. From a diving perspective - the dives on Oahu were not nearly as impressive as Kauai dives overall w.r.t. variety & abundance of quatic life, reef structures, etc.
The wrecks were "ok", with Mahi being the nicest wreck dive I was able to make on this trip. There are typically spotted eagle rays around the Mahi and during my dive, there were 7 "flying" in formation back and forth along the wreck - truly an awesome sight to watch.
All in all, I'll continue to dive and explore both islands on future trips. I saw a number of "cool" species (e.g. frogfish, lionfish, scorpion fish, ...) and average visability was in the 50 - 80' range on Oahu, and 75-100+ on Kauai, with water temp's a comfortable 80.