I thought Absecon Island (containing Atlantic City) might be on the list but its population is around 57 thousand so it seems to fall short. Maybe it's #10. Galveston Island Texas could be #11.
You are right. But I was researching a bit and I can see more than one Miami Beach islands. So I will add it if you or someone else can help me with this problem.
I did not realize there was more than one island involved. I am not familiar with the area. Good luck with the research. American Samoa is another candidate if you want to expand this quiz.