Percy could win because there is water everywhere. Even in the human, or demigods body. all Percy has to do is summon the moisture out of Jason's body and he will disintegrate. Also Percy can absorb the lightning with his water powers. He can create a tsunami that will drown Jason. He can do that wind thing he did with the wind dude in the fifth book. That would hopefully knock Jason over so he cant do any lightning bolt shazams.
Jason on the other had can cause a lightning bolt to hit Percy. He can create a shock wave for him. He can also use his awesome sword, and javelin thing.
Sadly in a sword fight, Percy is much stronger, faster, and smarter. He has more experience. Also he has the daughter of Athena on his side. In a sword fight, Percy kicks butt!