Potentially dangerous for the driver and fireman I’d have thought.
Should be OK as long as they can swim.
Swimming doesn't really work unless the water sits still, with a flat top and air above it!
And I think that's the key to what could have happened. We've seen the "spray" coming over the old sea wall, which goes up the wall and slightly forwards and comes down onto the track. And it contains some big lumps of just water, not mixed with much air, and capable of piling up quite deep. Of course it immediately starts responding to gravity by falling to the ground, tending towards the lowest and flattest it can.
So, could it - briefly - either sit there and have a train drive through it, or force its way in from the side, deep enough to force its way through the air damper to the underside of the grate? It's very hard to imagine - this kind of dynamic behaviour of water just doesn't match our intuition, mainly because it's hard to experience or even observe closely enough to understand it. But I suspect it is possible - perhaps not likely, but then that wasn't the question: it was reported as happening just once.