The problem with sea foam is that it's mostly air, so you sink in it. If they were experienced surfers in storm conditions, they were almost certainly on shortboards or mini-guns, which barely float them to begin with, by design. They would have leashes holding the board to their ankle, so they would have their flotation device but if the combination of board and body don't float, the leash won't help.

I suspect some light seafoam got sucked under denser foam by currents and then, as it upwelled, basically opened a hole under them. I can't say I've experienced anything at the scale that would drown 5 men, but falling into upwelling foam is definitely a thing.