DS2 wasn’t finished, but fully operational. The trap was the Rebels didn’t know DS2’s laser canon was functional.
How many unfinished but fully operational military vessels do you see out on the ocean?
It wasn’t really out in the ocean, it was still in dry-dock, as it were. It never moved, except perhaps to rotate a bit to point the superlaser. It couldn’t, without losing its shield.
Vader promises the Emperor that it will be completed on schedule, just as Jerjerrod promises he’ll double their efforts after complaining he doesn’t have enough men to do it in the given time. The surprise is that it’s operational - they got the cannon online before finishing the armour and probably the hyperdrive. Hence without an externally generated shield a few ships could just fly into the incomplete superstructure and knock out anything that looks vulnerable.