They're HOB, the motor/pump would be on the outside of the tank and the intake tube would in on the inside of the tank.
If your just talking about taking the pump and using it like a power head then yes they can be submerged completely. Though I've never heard of some one doing it because it needs to be connected to the impeller housing to work.
i was using it briefly to mix my ro w/ salt. i would just put the ac pump in the bucket. it would mix my water w/ no problems
i have noticed the impeller, propeller,whatever has some rust on it. that's why i was asking. but whether it's submerged or not, i don't think they should rust. i know many nano reefers use these filters as fuges, i wonder if they check the impeller for rust?