It's a big coast and the monitoring systems may be old and not suitable for detecting mini-subs. Much of the defense of the US was conceived for a completely different type of threat. We expected Soviet nuclear subs, not small battery powered craft that may not even be manned.

I don't suppose it will be long, if it hasn't happened already, before unmanned drones will be delivering most of the illicit cross-border shipments. You could cheaply build a fleet of them and overwhelm most coastal defenses.