<p>Percy can bloodbend, was equal to Kronos in combat, and has nearly killed a god. He is clearly the superior fighter.</p><p>
</p><p>However out of universe fights are not allowed so this will likely be locked.</p>

<p>I'd say it depends on their surroundings. Percy would firstly have to rely on his demigod abilities since all his weapons can be forced away from him with the Expelliarmus Spell (assuming he'd have no additional resistance against spells). That means he would have a great advantage when they are surrounded by larger bodies of water since he could control them and even form hurricanes. Harry wouldn't be able to deflect these forceful attacks all that good since he isn't as skilled in spells that could do just that. Mainly Transfiguration, Conjuration or Vanishing would be able to block out such forceful attacks, soemthing that Harry always struggled with. He is more skilled in DADA, so charms, jinxes and curses, and less in Transfiguration, Vanishing and Conjuration. But if we'd look at surroundings without larger quantities of water, Harry would have a clear advantage. Though Percy might be able to generate his own water, it takes quite a lot of effort on his side to achieve. The time needer for that could Harry easily use to fire spells which have an instantaneous effect. The problem here would be how Harry would confine Percy, so make him unable to fight. He'd have to block of all his abilities, something that a simple Incarcerous Spell couldn't manage. He could kill Percy, of course, through the Diffindo Curse, the Exploding Charm or the Blasting Curse, but I don't think he would do any of that (And don't come with "He could use the Killing Curse", no he can't since he wouldn't have the pure intention to murder his opponent and the curse would tehrefore not work). So Harry would have no way of defeating Percy without killing him (since there is no known way to take a demigod's power), which he'd be reluctant to do. That means the only way for Harry to defeat him would to use an immobilizing Spell like the Full Body-Bind Curse, but it is unclear whether or not Percy would still be able to use his hydrokinesis in this state. </p><p>
</p><p>So in summary: I think Percy would win when they are surrounded with water since Harry wouldn't have the ability to block his attacks. When they aren't surrounded by water the it'd either be a win for Harry (when he can't use his hydrokinesis when he's under the Full Body-Bind Curse) or a draw since Percy might still be able to attack, but not to free himself from this state since he can't use spells or brew potions to undo it's effect. </p>