Is fishing outside of the territorial waters legal ?

It is legal and common for commercial fisherman to fish international waters. But they are still held to the country of entry standards and regulations. However Some countries respect others countries regulations, and some don't. If you can prove you caught fish in Mexico, you can bring back a Mexican limit. But coming back from international waters you have to follow federal and state regulations.

90% of all fish caught are from outside territorial waters. It is your government that controls where and how you fish. For example, crab fishermen (US citizens) in the Bering sea, which is in international waters, are limited by a quota system and a specific season. Watch "The Deadliest Catch" on the Discovery Channel. They go out hundreds of miles. There are international agreements that control what happens in international waters. Some countries, such as Japan, still hunt whales and have not signed on to an agreement to limit their whale hunting, but have signed to limit certain fishing.