We cannot say. Powergirl's power levels are iffy. Since she got trounced by Superboy Prime, her power levels are still post. But they still fluctuate like mad.
If Power Girl's powers were stable and normal, than the odds are in her favor in several ways:
She's likely stronger.
She's likely faster.
She has heat vision...which is not good for an amphibian.
HOWEVER, she's stupid. She's incredibly rash and headstrong and has no tactical sense whatsoever. Her fighting ability is limited to a few boxing lessons from Wildcat. Namor has an enormous edge in this department. Power Girl is notoriously cocky. After seeing her superior strength over his, She'll start to get slop with self assurance of her victory. And Namor will go into the water. Superman would use heat vision to boil the seas, but Power Girl is likely stupid enough to follow. Thus the tide of the battle may change.
Even so, Power Girl (normal) would win most of the time.