Kayaks? No one pays any attention to them. Naval Bases do have some security so an attack on one of them may have its difficulties but if the terrorists wishchoose "civilian targets that will damage the USA the number of absolutely inprotected targets is too large to even imagine. As to a scuba borne attack. Rebreathers are unecessary. Who will pay attention to bubbles except at a Navy base. Even then the chances of bubbles being spotted on a breezy day is slim. Yes the USN has counter measures available but outside of US Navy facilities what security is there in the water? hey in New York or Boston Harbor if they do not not wish to use a 55 gallon drum they can substitute a refrigerator. As to swimmers in the water people as a rule don't even call in dead bodies in the water.
Stay well - Stay safe - Stay armed - Yorktown
Tight security in one area just causes them to move to one of the million other unprotected targets.
The key is their motivation, not their weapons, skills, or particular targets. Any former SOF person could take 20 motivated Boy Scouts and spread utter havoc.