You need to stack breathing buffs, so equip Crystal of Infinity – Submergence x 2 in Luck Shoes of Fortuna and buy Agile Seal Elixir, and seafood cron before diving for extra speed and breath time. Fortuna shoes are not usable on season characters, use an expendable naru or tuvala instead if you are season.There is an invisible dome on top of the ruins, so you need to go to one of the four entrances on the bottom of the sea nearby the quest markers.Place your boat on top of a marker and dive straight down, you will find it.