Perhaps with this somebody can translate it correctly. I understand the concept but not how to express it in Spanish. Good luck.

No sé cómo se diría en español, y dudo que haya una traducción establecida, pero aquí te dejo información que te puede resultar útil.

Espero que te sirva:

The term "water's edge" refers to this method of taxation which taxes U.S. corporations as a unitary business, but only on the income earned between the "water's edges." Thus, after a water's edge election is made, the foreign affiliated corporations (e.g., a Canadian or Mexican corporation) are not normally subject to California's unitary corporate income tax reporting and compliance system. The U.S. corporation must include other U.S. subsidiaries in its unitary business if that domestic corporation can be included in a U.S. federal consolidated tax return.
Also, certain foreign subsidiaries must be included if more than twenty percent of the foreign corporation's average property, payroll, and sales are generated in the United States.