What if you lived underwater, and could not breath air? Added features might be a mer-tail, underwater plant life for food, etc., fish that attack, and maybe some sort of water-bubble gun to create over-world 'fish tanks'. You could also add differing biomes underwater, and flood caves so you could mine them. Bio-luminescent plant-life could be especially useful down there.
An underwater mod could also have things like sunken ships, sunken treasure, and maybe mythology lore, like Neptune, or something. You could even add an air-diving suit with water tanks for limited surface excursions.