Two phrases that convey unease in the excerpt are an air of melancholy and sensing misfortune.
What are the phrases?
The first phrase is "so slowly and with such an air of melancholy that the onlookers, sensing misfortune, began to wonder if an accident happened on board." (Line 3-4).
This phrase directly describes the onlookers' feeling a sense of unease and misfortune due to the ship's slow and melancholic approach. It uses strong verbs like "sensing" and "wondering" to convey their apprehension.
The second phrase suggests a specific individual's heightened anxiety, so much so that he takes immediate action to meet the ship before it even docks.