To get to this point, one would have to look back to the moment Sewlynn told Su Yeng what he needed to do.
It was as just simple as he said; Su Yeng's only role in this was to climb to the top of the crow's nest aboard the ship and continuously flash a red light in a three-point rhythm.
There was substantial interference on the ship, just like the one Anson found on the pinhole camera, which prevented any signal from being intercepted. This technology is also what the vessel used to hide its location from their detector. At this time, Selwynn could not get any information in or out the ship.
This also made it harder for the Marine corps to pinpoint the location of the ship. By Su Yeng doing this, what Selwynn wanted to do was to form a communication with the coast guards when they arrived at the ship and allow for them not to move rashly and give him space to act.
Su Yeng's face turned slightly grey,
"But..but I'm scared of heights..."
"...."