The verb suspend has multiple meanings. This is a good example.
In terms of what time period the first sentence refers to, it's unclear without context. The suspension can either be ongoing or completed, depending on when that sentence is said.
Your second sentence uses a different meaning of the verb suspend. The first sentence deals with a punishment. A boat is not a person/animal, so the boat cannot be punished. When it says that the ferry service is suspended, that means that the ferry service is halted or not happening for a certain period of time.
Your inquiry about the day versus a day is a good one. The two suggest something slightly different. To me, the day suggests the rest of today, while a day suggests a day that is not today (either in the past or the future, depending on the tense used). The same reasoning applies to the suspension, which is why a week is used instead.