Sunscreen, a hat, and water are must-haves. Most tours include life jackets, but check your listing for what's provided versus what to bring yourself.
Yes — safety comes first, so tours get rescheduled for storms or rough seas. Most operators let you reschedule free or get a refund if that happens.
It's possible, especially if seas are rough, but most harbor and near-shore tours are pretty calm. If you're prone to motion sickness, talk to the operator about remedies or gentler options.