Interesting, why would they sail a nuclear sub on the surface in the straight? Would that give everyone its acoustic signature and so on?
In case you're being serious, subs sail above the surface to not give away their signatures like a stealth fighter flies with Luneberg lens to not give away their RCS.
The Russians sailed a Kilo class, surfaced the entire journey around the eastern Atlantic when it passed NATO navies, on its way to Syria.
Subs generally create a lot more noise when sailing surfaced. They can probably also intentionally create more noise when sailing subsurfaced just to fool SIGINT efforts but sailing surfaced is also there to be seen. It's more an in your face message since subsurfaced doesn't carry the same "image" and political messaging. Since we should remember that Chinese SSBNs sailing subsurfaced in the western pacific is relatively common and not newsworthy. If it wanted to be detected while subsurfaced by creating intentional noise, it could send the wrong message but sailing loudly surfaced at least makes sure they are found (which is what they want) and the message being sent is SSBNs may be used.