so you want to search for surfing (the sport activity on the water), which gives you related and unrelated results too, pick some related ones, check the other tags on those photos, check out the other photos of those accs & collect a few more niche specific hashtags, rinse and repeat this until you think you have enough, in this process you'll stumble upon authority accs, check out the tags they're using and/or you can also use hashtagify.me to find related hashtags
or just head over here: on the right you can see the related hashtags ordered by popularity,
on the left there are a long list of authority accs
easy pease