I find a lot of Seaking in the Safari Zone through fishing, but not a single one holds the hidden ability. Is there some trick to get it, or do I rely on luck?
Based on personal experience (I tried to get one of these) it's just luck. However there is a rumour that I heard that if you bring another Pokemon with the lightningrod ability, it increases your chances. It's probably not true so I wouldn't count on it. Good luck!
Hope this helped-