If they use public roads and bike paths/lanes then you have every right to do your own thing ride-wise. [EDIT] You'll end up riding with riders of all capabilities, including those who are totally clueless. It might pay to avoid the congested start area, maybe leave it until the 5km mark to join the route.
From all reports it would be an act of lunacy to try to use the ferries even if you paid for the journey due to the sheer volume of ppl and bikes trying to fit on them.
Have fun.