Shark finning has already been banned in the US for a while, and they don't go after finning boats in international waters.

CITES recently restricted international trade on roughly 90% of shark fins, so there's already going to be international cooperation on seizing shark fins from protected species in the ports of approximately 170 countries. There's no reason for the US to take things into their own hands in international waters when it will already be dealt with by border agents both in the country of the fins' origin and in the importing country.