Best to check with the manufacturer. I have AquaClears,maxijets and the globes--all are submersible and some have a minimum water line--ie, the water must be above that line. Most should be submersible, though.
I did some checking around on this not to long ago. From what i could find they are submersable, Its a matter of law, that states that they cant sell them as "Submersable". That's why there's water line. Same with Ebo and tronic heaters.
I just bought my Aquaclear powerhead for 20 gallons and the LFS said I can submerge it halfway down the tank so the cord is actually partially in the tank. It seems to be working fine...Is it suppose to make little bubbles? Its kinda strange though the instructions while saying that the piece is submergable and safe at the same time theres a warning on here that states that if the pump falls in the water it states not to touch it before unplugging it..so I dont get it? Isnt the pump suppose to be in the water..how would it work if it wasnt in the water???