I third not buying tickets in advance and not buying round trip. We just see what the next ferry leaving is and purchase for that. Not sure if you can buy tickets for all in advance anyway. Last trip, we showed our passports when buying tickets, but maybe you can do that after the fact, before boarding.
Kevin may be able to help if you find the people coming in will be delayed enough that you are cutting it close, getting everyone there smoothly or making sure you get the best available ferry. (phone # (340) 643-0025))
Enjoy your trip !