EXTREMELY fictional. There's no way in hell one of these things will actually go 450 miles UNDERWATER and hit a target--maritime targets have this annoying habit of MOVING.
Read up on Outlaw Shark, and the oopsies that happened with that program, to get an idea of (1) the isssues involved in OTH-T, and (2) why we canned the TASM. And the TASM could actually receive target location updates while in flight. Your supercavitation weapon can't.
So, you're stuck with either (a) shooting at targets that can't move at all, (b) shooting hundreds of these things for one hit (which would bankrupt anyone except the United States, and we'd have a hard time maintaining that rate of expenditure), or (c) get used to your weapons locking onto anything that's even remotely close to the intended signature (i.e., your own Super Panamax container ships full of rubber dog poop and whoopie cushions, oil and LNG tankers inbound to Shanghai, et cetera).
Half of your problem is that you used Harpoon for modeling: that game system's EMCON modeling is extremely poor. Communications are never affected by EMCON, and are never counterdetected--you can have a submarine ghosting an enemy surface group via periscope, transmitting detailed OTH-T data without the enemy knowing they're there unless they get a hit on the periscope via radar. It grossly oversimplifies the entire complexion of scouting and antiscouting. Reality does not work like that. It also heavily abstracts the midcourse guidance issue in a manner that makes hits far more likely than they would be in reality.
Harpoon II was good enough for the USNR and has some ardent naval supporters in the user community who have dramatically improved the data base engine and editor in Harpoon III. But I am not going to argue with you on that...if you want to argue, go on those boards and tell them how badly they have done.
As to maritime targets moving...of course they do. But in the confines of the Western Pacific, particularly within the 1st island chain, there are places you have to go if you want to get somewhere. Such a weapon can simply go there and wait. In addition, as I have stated, there are variants that get seeded and wait for the target to approach, others that get launched by air, etc., etc.
But, as I have always stated, the over-riding thing here is that this is a fictional novel series. Don't get so worked up over it. Despite the messages I unabashedly try and convey...the book was written for entertainment. If you don't find it entertaining...the answer is simple and I will not hold it against you in the least.