From the manual:
"LOW: The Harpoon cruises to the target at low (sea skimming) altitude. Low altitude is also known as a sea skimming profile because the Harpoon travels to the target area at very low altitude. While maximum range is reduced, enemy reaction time is also greatly reduced. Cruise altitude is 5,000 feet"
This contradicts itself. In game, Harpoon does NOT cruise at low sea skimming altitude. It flies at 5000 feet, or lower if launching aircraft is lower than 5000, until it aquires target.