And islands still have to watch for random boats from any direction. Right now in Vietnam there is a little outbreak where a group of Vietnamese nationals illegally crossed the border by a fishing boat from Cambodia to Phu Quoc island, trying to avoid the mandatory two-week quarantine. Then they boarded a plane, arrived at their homeland and tested positive. Immediately their steps were traced by police, whole Phu Quoc is under a mask mandate, 3 levels of contacts are tested and in quarantine. It really isn't about being an island. That's a myth, you've been lied to. If you want an example closer to your home, look how hard it is to seal a land border with Mexico. They actually cross by rivers, so water makes it easier.