I am sorry you found lice on your child during your cruise.
We were on the Magic for the WTBA in Sept. and found lice on our DD the 2nd week of Nov. Do I think it came from the cruise? No, because her school has been battling lice since the beginning of the year. She has had it twice already this year. Her hair is long and thick and it is a pain, so I finally found some tea tree oil to add to her shampoo and I check everytime I wash her hair. It is working so far and my next step is to cut her hair shorter.
How long was your cruise? I ask this because from my research on lice it takes 7-11 days for eggs to hatch and then another 10 days for those to reach full maturity to lay new eggs. Lice cannot reproduce off the human body, so if they happen to fall onto the carpet, they do not lay eggs and multiply. While it is true that they can live off the body for 24 hours, they need a human host to survive.
From your original post, it sounded like your daughter had many lice found on her head, whether they were adult sized or hatchlings, I am not sure. If they were adult sized, it is highly unlikely that so many walked from an infected persons head (or tiara) in a short amount of time.
What is more likely is that perhaps there was 1 adult that she came in contact with before your cruise that was a stoway (so to speak). 1 adult louse can lay 7-11 nits a day. The hair braiders may have missed these at the base of her scalp while they were braiding (I'm sure they were chatting) or the lice were so tiny at that time (newly hatched) that they just didn't see them.
I guess my point is that based on the numbers above if you had a 3 or 4 day cruise, then it is impossible that the lice came from the ship if she was infested (if it was 1 lice and nits, sure). If you had a 7 day cruise, then it is highly unlikely, since it takes that amount of time for the eggs to hatch. Even if the lice hopped on her head and started laying eggs the moment she stepped on board, they would not hatch until the very last day of your cruise and it would take a few extra days for the hatchlings to be visible without a super close inspection. If you had a 12-14 day cruise, then it is still unlikely, but possible, if infestation was found toward the end of your cruise. If you did a cruise longer than 2 weeks, then I would say it was possible.
No parent likes finding lice on their kids head. Believe me, I know - I felt like I failed some parental test when I found them on my own child. Then I realized that lice happen. They don't wait for a time when nothing is going on - in fact, I found ours the night before our DD's birthday party. Talk about poor timing!
I know your trip was ruined by this and you are probably looking for someone to blame. I know it might be hard to accept that it wasn't Disney's fault, but it more than likely wasn't.