YMMV:
Hawaii has the highest boat ownership of anyplace I have ever lived, so boat traffic can be a concern in a few locations, but almost all the shore diving is done in places where there is little or no boat traffic. Ala Moana Park is a one spot near a harbor, but is not a place I would recommend for others for shore diving, given the other options.
If you stick to the easier shore dives, you will likely never need to bring a float because the tourist shops will likely be out in force. Legally you might be required to drag a flag, but. If you ever actually need to bring a flag a spear fishing bullet is the only choice I would consider trying to tow along, rather than tie off. A safety sausage can easily start pulling in the opposite direction because wind direction, wave direction, and current are often not related in Hawaii, as unlikely as that seems before you come to Hawaii. Moreover, given the site, your safety sausage might be needed as a safety sausage if you get blown off the dive site (and exit) by current. Some friends still dive with cellphones in canisters even on shore dives.
To answer your question by not answering your question, I simply would not recommend shore diving in many other places without a local guide. (The sorts of places that would not have other people's dive flags already in place.)The ocean in Hawaii is vastly different than any continental experience. As an example, be aware that there are a bunch of locals diving in places subject to a current known as the Molokai Express. The Molokai express is so named because you can end up getting swept towards another island (Molokai) from Oahu. The Molokai channel is 32? miles wide, and with potential 15 foot swells (measured on the face) swimming to safety is just not going to happen, because swimming in 15 foot swells is impossible. Ocean swimming is hard enough even if you can actually see the horizon. When you cannot see the horizon because of the swells, well.. Guides/experienced locals know the way to avoid or use the currents (Two car dives are pretty common). Well worth the effort as Lanai Lookout is, it is simply impossible to manage without a guide the first few times. You should find a local guide for it, because a Giant stride off a 10 foot drop into 10 foot swells is something worth doing even if the dive was not amazing anyway. But the exit is not at all obvious, and practical matters, like rental cars will always be broken into if left parked, when beater local cars will not be touched, and how exactly you find your way to the entry point are tough to deal with. Blowhole is another fun for the crazy entrance exit with surge slamming you back and forth 15-20 feet in bubble whiteout conditions, that open up into an easy dive once you are past it, and s long as you go nowhere near the blowhole itself, which is deadly.
The North Shore, which has rotating array of dive shops which sometimes specialize in shore diving, is mostly out for the winter surf season.
A guy named Seth Bareiss used to have a site with detailed maps for all the sites, and have other great info, but his site is no longer online AFAICT.
As always, one option you might want to explore is the overall value of joining a dive shop's club, which if you do a ton of diving will pay for itself in airfill costs, and will give you cheaper boat trips. On those boat trips you will likely meet locals who would (likely) be glad to guide you around to their spots. Call ahead to a few shops and see what sort of packages you can swim for club memberships.
(Special insider tip: if you just want to do a bunch of diving, and don't care about the where as much as the how much, you might find that paying for a divemaster course is the cheapest way to get a bunch of diving done in a month. I know that it is a trick that plenty of Japanese month-long tourists have long since used. Pay whatever for the divemaster course, be willing to lug tanks and be a bit of a dogsbody on the surface, and you will usually get to dive on an all you can eat basis, including boat diving. Especially now with eLearning, you can do all the bookwork at home, and then just dive your brains out. I always take DM candidates to challenging spots as part of the lesson that Hawaii's local diving is just not the same as anywhere else. In fact, I end up taking DM candidates to spots that I would simply never take other customers to because of the risk factors.
