I'd say about 90-95% of the time, if you manage to get in a preorder in the first minutes preorders open, you'll get the order on release day, that is April 24th.
Actually, when you order, you'll get a "delivers by" date, and you can stop the ordering process if the date is after you leave the U.S.
Once you have a "delivers by" date, as I said, you will most likely get the watch on or before that date. But shipping accidents do happen, so I'm afraid there isn't a 100% guarantee. But it's very rare that a shipment doesn't arrive by the "delivers by" date.