These quotes from the poem are describing the effects of poison gas. The ‘five-nines’ referred to are German artillery shells which contain the deadly gas (probably chlorine, which was only one of several poison gases deployed during this war). When these shells land and burst near the soldiers the effects are instantaneous and horrifying. The scene immediately become enveloped in the green colour of the gas, so intense that it’s like a light covering the men. They scramble to put on their masks but one man at least gets the worst of it. He is described as ‘drowning’ in the green sea as the gas has a lethal effect on the body; it chokes the lungs, as does drowning, and the man is suffocating.
The description of the gas effects as 'a green sea' is very striking. It lends a strange, surreal air to the whole scene. To be under the sea is not a normal condition for human beings and to be under the effects of poison gas is not natural either. Owen, of course, along with other war poets, attacked the whole business of the First World War as an evil senseless event, which caused so much physical and mental suffering to the soldiers. In this poem and in others Owen deliberately provides a graphic picture of this suffering to hammer home the point about the utter destruction caused by this war. He does this as a corrective to the notion dating back to ancient times, that war is a gloriously patriotic and noble undertaking which elevates those who take part in it. This is the gist of the Latin quote that forms part of the title and also closes out the poem: ‘dulce est decorum est/pro patria mori’ (translated as ‘it is sweet and fitting to die for one’s country'). Owen denounces this as ‘the old Lie’ and instead gives us the grim reality.