I believe you’re thinking of Michael Dobbs’ article in the Washington Post. The related graphic did support the Swift Vets’ version of events, though the Post was careful not to point that out. However, there’s no evidence that Kerry’s boat (or any of the others) was fired upon during or after the mining incident. See the linked chapter for details...
After the initial mine explosion I can imagine that our boats may have started firing on shore at suspected targets. Once one boat begins firing that could induce the other boats to also fire on suspected targets. It could take a while to conclude that nobody is firing from shore.
Is this the likely scenario which gave Kerry the cover to claim that they were being fired on or do the others assert that there was no small arms fire?