Boating is such a normal part of life around Miami that it's easy to forget how different the safety math is compared to a backyard pool, no walls, deeper water, often a fair distance from help if something goes wrong.
A life jacket sized for an older sibling, or a generic "kids" size grabbed off a shelf, often doesn't fit a specific child well enough to actually work as intended. The jacket should keep a child's chin above water without riding up over their ears, if it doesn't pass that check on dry land, it won't perform better in the water.
Florida requires children under six to wear a life jacket on boats under 26 feet while underway. That's the legal minimum. A lot of families reasonably keep young kids in a jacket the entire time on the water, not just when the boat is moving.
A confident swimmer who ends up in the water unexpectedly, jacket or not, tends to handle the moment with far less panic than a non-swimmer. It's not a reason to skip the jacket. It's a reason to do both.
Comfortable in the water is a good complement to a well-fitted life jacket.
See Kids Program