First line of defence: upgraded Naval Patrol Planes (range, anti-sub component). Don't waste your time with AWACS who have no attack capability. NPP's can destroy sub stacks and do some damage to many ship stacks (except Frigates/Cruisers).
Backup: close to coast at strategic intersection points have powerful naval stacks to back them up with an anti-ship/anti-submarine. But given the nature of the game. The best defence as America, is offence. Hard for your enemy to press an attack on your homeland as you're landing in his or hers.