Aqua Guardian (MHFU name) is one of the most powerful water elemental Long Swords in the game, the strongest one being it's upgrade called Atlantica. Aqua Guardian becomes available as soon as HR7 is reached. It is recommended creating it instead of upgrading a lower form into it if you have the basarios tears because upgrading the base weapon into this is way more expensive and costs more materials then is necessary. Besides, the imperial sword also needs 3 basarios tears and costs quite a lot so you shouldn't hesitate to "create" it. You'll use the tears anyway, so use it for making it directly instead.