C'mon, people. Think this swarm thing through. If we are in an all out war with Iran which includes 20 mile free fire zones with no regard for civilian casualties and collateral damge, unlimited attacks on bases, ports, and small boat piers (again, with no concern for collateral damage), aerial sweeps for small boats, etc. then the swarm threat is no more than a minor nuisance - an afterthought. A swarm attack won't even be able to get started.
Unfortunately, that's the least likely scenario. Far more likely is that the US will either be ambushed with no warning or will allow itself to get sucked into a "limited" conflict with politically guided, proportionally graduated application of force coupled with extreme concern for collateral damage. In these scenarios, we won't be identifying and engaging everything in a 20 mile radius - we'll be forced to wait for demonstration of hostile intent (we won't fire until fired upon) and we'll be forced to obtain unmistakable positive ID of enemy boats before firing. Is that boat a civilian boat or does it have RPGs hidden under the usual piles of junk? We won't know until someone shoots. That swarm boat that we can see is beside a civilian boat - we won't shoot.
These are the circumstances in which swarm attacks will be effective.
I'm seeing way too many unrealistic assumptions about helos. An LCS carries one helo (the -2 version may possibly carry two although that's highly questionable and has never been demonstrated to the best of my knowledge). Helos are notorious for being down all the time for maintenance. Even when operable, they're only airborne for a few hours each day. The odds that a helo will be airborne, in the right position (not investigating something a hundred miles away), and appropriately armed at the moment of an attack is extremely poor.
Further, a helo can carry 4-8 Hellfires. So, a single LCS (or Burke) can, in theory, kill a maximum of 4-8 boats (realistically, probably only a couple) before it's out of weapons (assuming the boats have no shoulder launched SAMs which helos have been proven to be highly vulnerable to). That still leaves the rest of the swarm to deal with.
Helos are not the answer. At best, they may be useful as a small part of a layered defense. Most likely, they won't be available.
Don't just match up individual weapons. Instead, think through the overall scenario. Yes, a helo can kill a boat but in a realistic scenario will not likely be all that effective or available. Ask yourself what weapons the Navy actually has and what platforms would realistically be available. There's nothing wrong with "what we ought to do" suggestions but recognize that those are different from "what we can do".
Don't just spit out individual weapons. Think through the scenario. Reader GAB had a great comment about the perils of exposed personnel trying to operate anti-tank-ish weapons while receiving incoming rockets and shrapnel. Yes, in isolation, a Javelin or TOW or whatever can destroy a boat but what is the reality of the overall scenario? Can exposed personnel effectively aim a weapon from a high speed, pitching, rolling ship being showered with rockets and shrapnel? Can you even target a boat that is hidden in the waves or spray half the time? Sure, a weapon can pitch up to get a clearer view but it still needs an initial target lock. What is the effective target lock range under those circumstances versus the Wiki specifications for weapon range?
Circumstances, rules of engagement, and practicalities are going to have far more impact than weapon specifications so let's think along those lines. What I'm asking is that we think this through from a realistic perspective.