This is interesting because back in the days there was an anti-ship variant known as TASM. It was removed from the fleet in the mid-90s. It was *not* the same missile as the land-attack version. I can’t speak for the new one, but the old versions were not interchangeable; they were either anti-ship or land-attack variants.
A Tomahawk doesn’t cost a million dollars, has a greater range and is faster than a Predator. Each Predator requires a trained operator; a few people can launch dozens of Tomahawks. Tomahawks are launched from a ship’s Vertical Launching System or from a submarine, which you can’t do with a Predator. It’s radar cross-section is pretty small, and it would most likely be a sea-skimmer, so while anti-missile systems would be a threat, probably not much of one.
But a Tomahawk is used up in each mission, while a Predator can launch one or two Harpoon missiles, then return for reload/refuel.