The second paragraph paragraph introduces doubts about peacefulness at sea: "The daytime was serene, but dusk brought a change." The writer further states that there are indistinguishable dusk forms.
In the third paragraph, it tells us that the dark forms are ships.
This would most closely match with choice (2) because the writer describes a peaceful but potentially dangerous sea.
(1) is incorrect because the writer does not describe a battle
(3) is incorrect because the writer does not describe an attack.
(4) is incorrect because the writer does not describe people talking
(5) is incorrect because the writer does not discover that there are ships and already knows there are other ships: "present only in our knowledge"