Farming, fishing, and finance were the major industries of Jesus' day; many of these were family businesses. That is why almost all of His parables are based on these themes. At least four of the disciples were fishermen and everybody in the Galilee province was familiar with net-fishing which took place at night with weighted nets supported by floats, to produce a vertical mesh in which the fish were trapped. However not all the fish were good for eating; some were thrown away or burned.
 
Jesus was teaching the apostles about judgement. The kingdom of heaven's activity on earth is to draw people towards Jesus. He has the right to all the 'fish in the sea', the people who He has made. Some are fit for His kingdom and some are not (according to how they have responded to the gospel of Jesus). By the time that Jesus returns, those who do not love the King have no place in His kingdom, and they are consigned to hell.
 
Jesus does not minimise the reality of hell's agony. It is a place without anything good, and with no prospect of salvation. The angels will sort through the mass of humanity, one person at a time under God's direction, and will make no mistake. The blazing furnace is an even stronger picture than the continuously burning rubbish pile in the Valley of Hinnom (Gehenna). It suggests the intensity of God's wrath which cannot be quenched. The focus of this parable is on hell: a real, terrible but avoidable fate ... if only people would respond well to the gospel.
 
The largely secularised West, and the internet which easily spreads such values, has become a virtual space devoid of reality: a place in which success is greeted with praise and failures are eliminated. However a secular mind-set has no understanding of eternity or of God's wrath, they just live for the present. Jesus taught His apostles to teach about the long term consequences of faith and foolishness. Yet today, hell is rarely taught (as except as 'exclusion from heaven'). Trivialise hell and you take urgency away from the evangelist's heart and eternity from the pagan mind. So be true to the gospel, hell is far more terrible than warning people about it.