30 minutes early is the sweet spot.

Arrive 30 minutes before departure. That means 10:30 for an 11 a.m. Hornblower tour.

Boarding usually opens 30 minutes prior. If you show up at 10:30, you’re near the front of the line, not standing around waiting. If you come at 10:45, the queue is already forming — people who arrived early have been holding their spot, and you’ll end up behind them. If you come at 11, you