An island the size of Lulusia is capable of raising the ocean 1 meter when it's knocked down by Urunus, but, Pluton, the terraforming earth Ancient Machine, can also create ocean rise by producing earth to displace the water.
The Red Line goes down to the ocean's depth.
IMO, what drowned the world was the creation of the Red Line causing a massive two sided Tsunami.
This is why the Ancient Robot and Zunesha keep apologizing; Pluton is HUGE, and would have needed some big escorts.
The world flooded after the WG stole Pluton from the Ancient Kingdom, and created the Red Line through terraforming. Zunesha and the Ancient Robot were meant to protect Pluton, but failed. The Red Lines creation displaced a shit ton of water, separated Joy Boy's forces by splitting the world in half, and this displacement of water led to massive, world sweeping Tsunamis.
So, I think a lot of the water rise came from Pluton creating the Red Line, and the remaining water rise comes from Imu using Urunus to knock islands down to the sea floor via Gravity wells/fields, which suck all the light/energy up and send it straight down (this is what happened to God Valley).
In the past, Imu had access to plenty of Mother Flames, but, now, Imu has to search for them/pick and choose when to use Urunus. The last time Imu used it before Lulusia was most likely God Valley, to save the CDs from Rocks.