It really doesn't need to be though. Let's say he has a shitty little rocket.

Ok. So now lets shoot the rocket at the warship. Ok, it didn't do much damage. Maybe it was even shot down by anti-air defenses. Now try shooting 900 of them at the ship. They're cheap, they're disposable. Sure, the anti-air might shoot a lot down, but you're shooting down a $3 rocket with a 1 million dollar anti-air missile.

Even if you never hit the ship, you're causing financial drain that cannot be sustained. You spend $10,000 on $3 rockets, fire them all off all day and all night, and the USA is using military grade defenses. That $10,000 might drain the USA half a billion dollars just to defend one ship.

Now scale that for every ship in the military. Little tiny unmanned rafts basically, shooting off cheap rockets at close range, and being defended by expensive ammo. All across the ocean. With the proper planning, you could probably drain the USA's global navy in a day without sinking, or even hitting a single ship. All without losing anything at all.