On tranquil turquoise waters off the coast of San Diego, the U.S. Border Patrol (search) searches for possible threats to national security — by sea.

The Border Patrol’s Marine Interdiction Unit (search) also has the task of stopping illegal immigrant smugglers and possible drug runners trying to get to this country by ocean.

Since Sept. 11, security has tightened along the land border between California and Mexico, but the waters off San Diego and Tijuana have become a growing concern, as some terrorist threats have moved offshore.

The MIU monitors waters up to 12 miles off the coast of California and at least 30 miles north, nabbing surfers, jet skiers, boaters and even swimmers trying to get from Tijuana, Mexico, to the American beach.

Its patrol vehicle is a 25-foot Zodiac boat that can clock 45 miles an hour and cut through ocean waters fast enough to hunt down nearly any vessel in that part of the Pacific.

Click here to watch a report by Fox News Channel's Adam Housley.