In wartime the USCG along with the Reserves assume ASW duties in coastal waters. Not sure what the Navy could assume. Perhaps long range deep water surveilance to augment Port Security.
That's all in the Flag negotiations.
Perhaps in WW2 the USCG was as prepared as the USN to conduct ASW operations but in todays world, the USCG has basically no real undersea surveillance capability, especially when compared the the USN capability. This is part of the Admiral's point I think. USN would have to be part of the homeland security mission if we perceive certain threats for which the USCG is unprepared, but in a similar manner the USCG has a level of physical presence around the coast that the USN could never hope to reach. In a defense in depth approach, both services would need to cooperate pretty extensively to really cast a tight net around the coastal borders.
They would probably slow us down.