Queen Mary is not a small ship, it's better to have a more specific place to meet (a bar or deck).
==>"九月12号:在洛杉矶举行网友的中秋聚会。如果有家室的网友不方便出来,可以改为11号"
Anyway, here is may take of the day:
* 9/11 is Sunday; probably better for everyone but not sure if reservation is needed (since the date could be some sensitive)
* 9/12 is Monday and traffic to Long Beach could be very bad (Long Beach is a very busy harbor) for lot of people living in San Gabriel area
So, I vote for Sunday the 9/11. Late afternoon (before dark) is probably better, since all can enjoy the nice view from QM.