The others on the pier don't seem to concerned about anything..
Looks more like this painting should have been called, "The 'Oh shit, I think I left my wallet on the bench back there'."
They could be looking the other way, but I always understood the scream to be internally anyway. Maybe due to existential crisis... or due to a forgotten wallet.
Maybe they stepped in dog shit and now they feel like a dog... I'm getting flashbacks to the torture that was school... maybe that's the point of the scream... AAAHHH