It is way too expensive to keep choppers flying all the time. And they are way too long to take off compared to the speed of the threat. Actually anti-missile cannons or missiles like the aster 30 could be "upgraded" to tackle speed boats. Lotsa firms already working on it. But you can't have one on every tanker. So in many cases, you can only react after the boat has been seized, and that means a heli dropping very, very mean troopers on the boat, and providing them "support" (meaning: shooting anything that moves on the deck)

Pierre