This shard could be from some piece of pottery that fell into the sea when a ship sunk but it does look more like it's a beachcombing find rather than a diving find.
All the corners are rounded in my opinion by the rolling action of waves, sand and pebbles, maybe by the wind and sand acting like a sandblaster on some beach it landed.
I've picked up a lot of similar shards just for the fun of it and they sure look like this one...