The notion that some guy standing on the bridge periodically glancing at the scope is enough is simply not realistic.
You are right, a ship would need a dedicated security watch with decent sensor equipment (I’d go for thermal over floodlights - floodlights help the pirates find you...).
Then they would have to decide what was a potential “mothership” and go on heightened alert - with many potential false alarms.
Its up to the company to decide which is better - paying ransom for a ship and crew, thus encouraging more pirates, or paying for a security watch and sufficienct armament to, hopefully, make life dangerous for pirate wannabes.
The good news is that Zodiacs can’t cross that much distance, and they’ll have some issue finding a ship at night, too. And, if you spot a zodiac approacing, you KNOW its up to no good - no warning shots required...