Upvote:2
I wonder if there are usually spaces on evening ferries ... for travelers without reservations.
It seems that the chances are reasonably good. It would be prudent to have a backup plan though.
esqew commented
I won't post as an answer b/c I don't have any official documentation to show for it, but in my experience: Cross Sound Ferry is particularly flexible with ticketing. If you're running late, you'll likely be able to call ahead and change your reservation. If you arrive at the terminal early, you'll be able to "stand by" for spots freed up by no-shows. (Neither of these methods is guaranteed.)
Michael Seifert commented
It's also worth noting that there are scheduled departures at 8, 9, and 9:30 that evening; and currently (around noon on a weekday in July) there are still 42 spots available on tonight's 8 PM ferry.
Footnote:
I'm posting this as an answer mainly because the question asker decided on a course of action and this question doesn't really belong on the unanswered list. It has been "unanswered" a few months and got no answers other than the material in the comments.