International waters is not a law-free zone. If your ship is flying the flag of any country, that country's laws apply aboard the ship, and that country may board you or authorize another country to board you. Also, the people on the ship may be subject to thejurisdiction of more countries, For instance, if you're running an operation that smuggles drugs into the Philippines, you are subject to the jurisdiction of the Phillipines, and the Philippines can ask for your extradition. China will also have prescriptive jurisdiction, because the perpetrators are Chinese nationals (your country of citizenship has jurisdiction over you everywhere in the world; not all countries claim that, but they can if they want to). The only thing they can't do is board you; the flag state ordinarily has the exclusive right to board or authorize the boarding of ships flying its flag in international waters, with exceptions for pirates, slavers, unauthorized broadcasters, and ships which a navy has grounds to believe are stateless or are really the same nationality as the warship (in which case they can board and ask for your papers). The Philippines and China could ask a flag state for permission. Oh, and UNCLOS provides for a contiguous zone as well, which is 24 (not 12) nautical miles off the baseline, where the coastal state can exercise some level of control for several purposes, including enforcing customs regulations.
If a ship is not flying the flag of any country (or flying the flags of multiple countries), which included if it's stateless but flying a flag without any right to do so, then every country is permitted to board the ship and enforce their own laws. There's some dispute over whether normal jurisdictional rules totally stop applying, but one thing which is certainly true is that the Philippines or China could board you (since they would normally have jurisdiction anyway). Other countries (like the US) take the position that any country may treat stateless vessels like its own, and apply its own criminal laws to stateless vessels purely because they are stateless.
There are other approaches. For instance, it'd be legal to monitor the drug ship and track its boats in, stopping them when they enter the contiguous zone. If your customers routinely get arrested and sentenced to many years in prison, you will lose your customers. But international waters is certianly not a place where you can act with utter impunity.