Note that if you want to go to the Pearl Harbor museum, you can go at any time, it's free and you can just walk in.

If you want to go to the Arizona Memorial you will need a ticket. If you can't get a ticket for the time you'd like, at 7am HST the day prior they make unsold tickets available for morning time slots. I had reserved for 2pm and was able to get tickets the day before for 9am. I believe the tour companies hold a lot of morning tickets and they release them back for distribution if they haven't sold.

Unfortunately, there is no way to cancel the tickets you won't be using. I tried and there's no option. I even tried telling them at the gate and they didn't care. BUT, if you end up doing this, you'll have to open a second account with the NPS. The system won't let you have 2 sets of tickets for the same day. It makes no sense, but that's the way it was last month. I had to scramble to set up another account at 7am. If you can't get a desired time, have a second account ready.

They also have standbys on the same day. I wouldn't take my chances with those, particularly if you're going at a busy time.