I'll take a stab at answering your Hilo questions... I live and dive here.
There are currently 2 dive shops operating in Hilo. Nautilus Dive Center has been in operation the longest, and was where I got certified (in 1976
). It is a small "mom & pop" LDS, but the owner (Bill) is an "expert" on Hilo-side diving, and can give you good advice if you decide to dive in Hilo.
There are no dive boats that I am aware of regularly operating out of Hilo. I believe a company called Lobo Del Mar does do some limited runs, but nothing regular that I've seen.
Why? Weather. Hilo is on the windward side of the Big Island. This means we get the wind and rain when the Trades are blowing (they blow year-round). There are some good dives, but they are very weather-dependant.
Leleiwi is probably the best dive spot close to Hilo (just a few minutes from downtown) and is a good shore dive with a fairly easy entry off a low lava ledge. It is a good multi-level reef dive, lots of large green sea turtles (I usually see over a dozen each dive), cleaning stations, reef fish, eels, nudibranchs, the occasional spinner dolphin pod.
There can sometimes be a deceptively strong longshore surface current at Leleiwi, but no current at depth (so navigation is important... you want to return to the entry/exit underwater if possible to avoid a tough swim against current). The reef is a very short swim out (30 yards), and goes from about 10 feet to a bit over 70'. Viz and water conditions are variable. While you can probably dive there 90% of the time, the viz and surge can make the dive conditions poor (compared with Kona). Viz probably averages 50-60 feet, 80-90 feet on a rare day, but can drop to 10-20 feet if there is a lot of surge. Kona divers on the other hand would consider anything less than 60 foot viz to be "low viz diving"
I typically recommend, unless the diver has lots of time on the island, to plan on diving
exclusively in Kona. The water conditions are good-to-excellent all year, there is the Manta Ray night dive (awesome, don't miss it), and TONS of dive spots from both boat and shore to explore.
Enjoy your trip