"Same as always. Go aboard and interrogate them. If it's a slave ship, kill them all. If only a few individuals were tempted by profit, kill only those who knew."
If these people had been Pirates rather than a smuggling syndicate, Nereus would not have sent anyone to investigate. He would have killed everyone in the Pirate crew.
It was not only because Pirates and smuggling syndicates differed in nature, but more because Fish-Man Island still flew Nereus's flag.
As a two-way passage, Pirates from Paradise entered the New World, while Pirates from the New World returned to Paradise for all sorts of reasons.
Perhaps the Pirates of Paradise did not know Nereus's name, but that flag was enough to restrain quite a few Pirates from the New World.
Of course, only some of them.
Before sufficient profit, even death could not serve as a threat. Before enormous gains, there would always be people willing to take risks.
When an island flew someone's flag, it meant that island was under that person's protection. This was an unspoken rule of the seas.
If other Pirates still dared to cause trouble under such circumstances, it could be regarded as a declaration of war. No matter the reason, even if only to preserve one's dignity and authority, they had to be dealt the harshest blow.
Anyone capable of running overseas smuggling these days would not have clean hands, but they were still very different from Pirates. If there truly were people who knew nothing about it, they could be spared.
Fish-Man Island did not need the reputation of slaughtering innocents. That would do no good for its future development.
As for how to interrogate them, making the survivors identify one another under the threat of force might involve some errors, but it would