no need to be catty, i dont think a wall is feasible either. but its not this great impossibility to find illegal migrants on the ocean, is all. they are often extremely overcrowded or have other suspicious signs (cargo ship with no cargo, fishing ship with no gear rigged, etc.)
people who do this stuff are trying to go for maximum profit, and dont care if the people make it or not. they already got their money. many times they will tell them how to steer the ship, then say good luck have fun.