If you are doing one of the very popular sites, then I wouldn't bother hauling your own flag. This applies to Ulua, Black Rock, Old Airport. Technically you would be breaking the law at Ulua and Old Airport, but they are well enough known as scuba diving sites, and there are enough flags already out there that not hauling one is IMO a reasonable risk/reward tradeoff. I'm not positive, but I think that the area in close to Black Rock has been buoyed off and is a no-entry area for boats. Few, if any divers use flags there.
When going to less often dived sites, then a flag becomes more desireable.
If you are experienced enough to solve problem without surfacing, and know your air consumption and can control your dive so that you are back to knee deep water when you surface, then a dive flag doesn't add much safety.
The law is rarely enforced, except at beach with lifeguards, like the Kamaole Beachs. Since I gear up at the car, head directly to the water and submerge, and do the same when exiting, I did a half dozen dives over a week or so at Kam III before the lifeguards managed to stop me, and this was with them on high alert trying to catch me after the first day.
No fine. Just told me to haul a flag.
You can rent flags at most dive shops.