Diving on the Big Island of Hawaii
Most of the independent dive operations on the Big Island go out of one of the harbors in and around Kona-Kailua. This is unfortunate since the diving is generally better further north up the coast. The dive operators work from Kona because this is where the harbors are and most of the tourists stay in the Kona area.
There are a number of luxury resorts along the Kohala Coast north of Kona. These resorts generally all have their own dive operation. You can often go diving with these outfits even if you're not staying at the resort. So if you find yourself staying in Kona, you might want to take a day to go further north for a change of pace and to see what you're missing.
The water temperature in these waters varies between the low 70's in the winter and spring to the high 70's in the summer and fall. Visibility can be as high as 120 ft, but is typically closer to 50-70ft. There is occasional surge current, but I've never run into strong steady currents. Most diving is between 20ft and 60ft, with occasional forays to 80-90 ft.
Nitrox is available at Kona Coast Divers (in downtown Kona - I don't recommend their dive operation, although I have no personal experience) and Kohala Divers (in Kawaihae, north of the Kohala Coast resorts).
Dive Operators in Kona
There are three dive operators in Kona that I have personal experience with. From what I've heard from divers living on the Island, these are three of the best.
Easily my favorite is Aloha Dive Company. Mike Nakachi knows all the best dive spots within an easy (or long if you're willing) boat ride from Kona. Tell him what you want to see and he'll most likely find it for you. Mike and his partner Earl Kam operate a small 28' boat with no more than 6 divers so you always get very personalized service. If you know what you're doing, Mike is very flexible with dive profiles, etc.
One of the dive operators with the best reputations in Kona is Dive Makai. They're a considerably bigger operation than Aloha, and usually have two groups of divers on the boat. They have some of the best underwater naturalists that I've run across and give an excellent briefing before the dive. You don't get the personalized attention that you get with Aloha, but Dive Makai is definitely a first class operation none-the-less.
Jack's Diving Locker has both a dive shop (definitely one of the best, if not the best, on the island) and a dive operation. The dive boat is a little crowded for my taste, but the crew is friendly and seems competent. I only went with them once on a late afternoon dive and then a Manta Ray night dive. The afternoon dive was average at best, but the Manta Ray dive was definitely a worthwhile experience.
Kohala Coast Resorts
As you head north out of Kona, you'll go by several very nice resorts. These resorts tend to be more expensive than staying in Kona - some significantly more expensive. I've only had the opportunity to stay at two of the resorts, the Mauna Lani and the Mauna Kea Resorts. The last half dozen times I've been to Hawaii we've stayed at the Mauna Lani since we've decided we like it the best.
Mauna Lani Resort
The Mauna Lani is our favorite resort in Hawaii. They have traditional hotel rooms, as well as fully furnished one, two, and three bedrom condominiums. The condos are particularly nice with kids.
I did my first dive here in the summer of 1996 - a resort course which consisted of a short orientation lecture, a dive in the pool to get used to the gear, and a shore dive with the divemaster. After that, I was allowed to go on boat dives as long as I was the only uncertified diver on the boat and I stuck close to the divemaster. In retrospect, I would recommend going through an open water certification course prior to doing more than the shore dive.
Diving at the Mauna Lani is run by Mauna Lani Sea Adventures. They own the franchise for diving, snorkeling, whale watching, and sunset sails with the Mauna Lani and the Orchid hotels. The divemasters are competent dive leaders and instructors and are all very friendly. They're also quite helpful in finding interesting things to see and photograph.
The dive sites are all within a very short (5-10 minute) boat ride from the dock (which is located within a short walk from the Mauna Lani hotel and the condos). Since the boat ride is so short, the boat returns to the dock between dives allowing access to the hot shower, bathrooms, snacks, etc. A two tank dive is schedule for the mornings. You can also go out after lunch for another one tank dive, although the visibility tends to be a bit less in the afternoons. A night dive can be scheduled whenever two or more divers want to go out. Some of the most interesting creatures are only seen at night, so it's usually well worth it to try at least one.
The only downside of diving with Mauna Lani Sea Adventures, and other similar "resort" dive operations is that the average diver is less experienced. The divemasters are therefore compelled to be more conservative - the dives are all led by a divemaster who expects you to stay nearby unless they've become comfortable with your diving. Once the divemasters become comfortable with your diving skills, they are more flexible and will allow you a little more freedom.
Most of the divemasters like you to come up after 60 minutes so they can stay more-or-less on schedule. However, I rarely come up before 60 minutes and have logged dives as long as 90 minutes without the divemasters getting annoyed.
The Mauna Lani does not have Nitrox available. However, Kohala Divers is only about ten minutes away by car and will rent Nitrox tanks. You can also consider diving with Kohala Divers (their dive operation is called Ocean Sports), although they generally come back down the coast to the same dive spots that the Mauna Lani boat goes to.
The quality of instruction at Mauna Lani Sea Adventures is very good. I completed my Rescue Diver certification with them, and my wife and son also took courses here.
The Mauna Lani resort is situated next to some ancient Hawaiian fish ponds. These ponds are now used to raise endangered sea turtles until they are about two feet long when they are then released into the ocean in front of the Mauna Lani. As a result, the waters near the Mauna Lani have about the highest density of sea turtles anywhere. Rarely do you complete a dive without seeing at least one.
Octopus are also prevalent in the waters around the Mauna Lani. The divemasters will often find one during a dive and pull it out to allow the divers to feel it's skin texture - not a practice I condone but it is very exciting for divers that have never seen one.
All-in-all, the Mauna Lani offers fairly conservative diving with some of the best dive locations in the Hawaiian Islands. The swimming and snorkeling off the Mauna Lani's two beaches is also excellent. The accommodations are first rate and the service is excellent.
Mauna Kea Beach Resort
My wife and I stayed at the Mauna Kea Beach Resort in the summer of 1997. Mauna Kea is a luxurious resort with nice rooms. The room we stayed in had extra wide sliding doors out to a balcony overlooking the ocean, and was large enough to easily accommodate a king size bed and a sitting area with a couch. They had excellent maid service that brought towels twice a day.
The resort has several restaurants and bars. One of the open air bars served hors d'oeuvres in the early evening and usually had live entertainment. On most nights, we ended up eating a light dinner there.
The Mauna Kea has a very nice beach, but the snorkeling off the beach wasn't very good unless you swam out quite a ways. There is an area north of the beach where lights are shined into the water at night, attracting plankton. On most evenings, we saw a couple of Manta Rays feeding and swooping around.
Mauna Kea doesn't actually have their own dive operation. They have a shuttle service to Kohala Divers which is located in Kawaihae, a small town about ten minutes north of the resort. Kawaihee also has some nice restaurants perfect for lunch after diving (and significantly less expensive than the restaurants at the resort). Kohala Divers now has Nitrox available.