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.
Even with heat vision, she couldn't boil ALL of the ocean.
If Namor lures her down under, he likely has enough power advantage.
If there wasn't water around, she has power edge.
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"I'm not smart so much as I am not dumb." - Harlan Ellison
Well...I know she has distraction advantage.
And with her hairstyle, she resembles a more buff - and front enhanced - Sue Richards. He may propose to her, to be his queen!
__________________
"I'm not smart so much as I am not dumb." - Harlan Ellison