Perhaps it would be cheaper than sonar on coast guard vessels to have an active SOUSUS line of stationary sonar devices around the coast.
It takes hi-frequency sonar to spot small objects. Sousus was a passive system. Hi freq stuff has short range but good definition.
I just think the best way to kill the dragon is a blow to the heart, it will sprout heads as fast as we can chop them. Even with a 99% catch rate, 1% can kill a city in the days of nukes. Proactive instead of reactive military force is a better option. IMHO