Deff not pennywort, as the leaf shape and growth habit are all wrong. My guess is Shinnersia rivularis, but having never been inside a petsmart, I can't say how likely you would be to find such a plant there.
Emersed = rising above the surface
Submerged = underwater
Emersed growth of H. difformis :
Submerged growth of H. difformis:
I know from personal experience that PetSmart does sell H. difformis in emerged growth form. It grows faster that way, thus it's most likely much cheaper for them to buy. The old leaves that grew above water will eventually die off as the new submerged leaf form grows in, although the first few new leaves will most likely be "emersed" form as well.