Being an ocean demigod, Hal's primary ability is hydrokinesis, or control over water. He can manipulate water in any amount that he can see, think waterbenders from Avatar: The Last Airbender. That's pretty much his primary powerset. He can access water through walls and sense it to some degree if it's nearby. He's most powerful in an ocean, of course, but can work with freshwater, rain water, or even fog or humidity out of the air. He has mild control over water in its gaseous form, and being an Irish demigod, he can only control ice/solid from the first frost through the first thaw of winter in Ireland, regardless of where he is in the world (for example if he was in Antartica in June, in the middle of the southern hemisphere's winter, he would have no control over ice due to it being summer in Ireland.)

In addition to his watery powers, as a warrior demigod he is much less vulnerable to injury or earthly poison than most mortals, and he's likely to live at least twice a normal human's lifespan. (Technically potentially longer than that even but there's some weird fey promise stuff going on.) He also is supernaturally strong and agile, and can breathe and speak underwaterr as if on land. He's able to understand the communications of any creature that lives beneath the sea, especially those that are magical in nature. With animals/unintelligent creatures, he can loosely understand needs or intent, and sort of get them to understand his own, but it's not like he can suddenly talk to fish like they're people. (There's weird fey loopholes to this ie he could not communicate intent with a sea turtle because it's born on land, but he could understand someone regardless of language if they were born in a submarine or something.)