Which sentence is punctuated correctly? without water - the plant will die.

Without water, the plant will die. A comma is necessary between the two clauses to connect the sentences. Hope this helps :)

So I would also agree that a comma is needed.

Without water, the plant will die.

Both of these clauses need help. Without water is an incomplete thought and if we say the plant will die, while it makes sense, there is still that unknown of why will the plant die. By adding the without water you are completing the thought the plant will die.

Hopefully this helped and good luck