Under state law, potato guns that use combustion (instead of compressed air) to launch the projectile are “firearms,” and one with a bore of over 0.5 inches is a destructive device. Pneumatic potato guns that use compressed air are legal as long as they are not used like a weapon (e.g. shot at a person, etc.), so a line launching device would be legal under state and federal laws.
However, you should check for local city and county ordinances because some local governments prohibit use of any devices that propel projectiles. If you intend to use a line launching device on a state beach, you may also want to consult State Parks. And if you plan to use one to fish within a National Marine Sanctuary, we suggest you check in with that Sanctuary office to be sure they do not prohibit these types of devices.
As far as using it to cast a fishing line, nothing in the Fish and Game Code or its implementing regulations prohibit using a compressed air launcher as long as the fishing line remains attached to a rod and reel, or the person is brave enough to hold the other end of line in their hands!