I'd go to wicket at 7-7:30, buy tickets, walk to dinner and return for last cruise of day.

When I last went in October, I arrived just a few minutes before departure time, and the line was already quite long. I couldn't get a seat on the top deck, which is better. But I really didn't care about that, as I've done the boat ride many times before. For a first timer, though, I'd probably want to get there about 30 minutes before, in order to get a top deck seat. I'm not sure in August, though, as I've only been in May and October.