Been in and out of there in heavy swell often enough to recognise the following:
a) With swell above 4-5 m as looked to be the case, and at or near low water, you are better off going 3 miles east to Guetaria, accessible in all weather, or a bit further west to Ondarroa, also all-weather. If you MUST go in, in those conditions, wait for high water.
b) Don't go so near the breakwater, even with swell of 3 m. Better to stay a bit further east, maybe 80 metres or so, because even with lower swell the waves can break just off the breakwater head. Might not have helped these guys, but I feel it could have -- the seas break with less virulence a bit further east. Right in near the blocks is where they break with most violence, producing a forward white water flow that is travelling fast, and so dangerous.
c) Any swell above 5 m -- don't even try it, unless you are prepared to lose the boat.