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For most of the early game, you'll be walking the ocean floor, tapping your sonar button until you get a hit on some treasure, then closing in and taking that treasure. Then you take it back to the surface to evaluate and sell it or give it away to someone who needs/wants it. you should also be careful, as you can sell items needed to complete optional side quests, and even progress in the story. So only sell items that you have more than one of. Eventually you'll start finding shipwrecks, which you explore with full 3D movement, and you'll look for treasure inside those too.
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Although the game itself is actually quite repetitive, it never becomes boring. Finding a bunch of mysterious items and finding out what they all are is pretty cool, and the game has a great atmosphere too. Inbetween dives all your quests, shopping and item appraisals take place in the two small towns on the island. These bits add a lot to the feel of the game: they're always sunny, the people are all friendly, and the whole experience just feels really warm and happy. Interestingly, references to things like romance, alcohol, death and so on, that often get removed in English translations of videogames are left intact here. (And by death, I mean in the sense of an old woman missing her late husband, and so on, rather than the usual kind of videogame deaths that only serve as catalysts for more conflict.)
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I thouroughly recommend you play this game if you have the chance.